13Y 11 11,DER
11
B.\hCKOFT
The Problem of Solarization. Part I ,,
1 he literature on the \u\)jcct of solarization or the re-
vcq-sal of the photograpliic. irnaqe is so scattered and t o a cer-
tain extent $0 contradictor! that it hai seemed wise t o give a pretty thorough idea o f the problem in two papers and t o imstpone till a later papcr the application of the theor?- of C;rottliuss to tliii problem. I Iierc is an excellent preientatiori of certain portions of thc subject to tie found in a lecture by llajor-Gcneral \\'aterhouse,' whicli i w s delivered on .April 1 4 , 1901, and from 11hich I quote extcnsil-ely. I ' 'l'hc ohservation of the rcversal or apparent clestruction of the inipressed photographic image was a \-cry early one i n tlie histor!. of photography. J k l y in 1 x 0 1 , Iiittcr \ I i o i i ctl that silver chloride, hich liacl alrcadJ- been darkenctl in the violet ra! 5 o f t h e spectrum, twxriie lighter again 11 Iicxi cxposed to tlic > elloiz, red, and eipecially the infrare11 raJ.5, arid he attri1)utetl thii to a n oxidizinq action of t lie le\\ refrarigihlr ra!.i countcracting the deoxidizing ac\\-ollaston,' on the other tion o f tlie more refrangible rays Iiand, sliortl? aftcrit ards, in I P o i , stated that about the same time as Rittcr, lie had found t h a t ~vhilegum guaiacum lxcatne green 1)y oxidation \\hen exposed t o the violet rays, it was deoxidi/ed and brought back to it5 original yellow color b y tlie red and orange ray\. The same decolorizing effect could, Iione\-er, also lie produced by heat. ;Ilucli later, in 1840. Sir John Herschel' clescrihed a similar expcrit n m t in which, again, paper coated with the alcoholic tinc-
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ture of guaiacum \\as first exposed t o light. and then exposed in the spectrum, the reiult being t h a t the part acted on by t h e more refrangible rays became of a darker blue than t h e original ground, n-hile about the orange rays the original pale yellow color of the paper n a s restored. “ I n 1842, J. IT‘ Draper, by allowing daylight t o fall along with the spectrum upon a Daguerreotype plate, found t h a t the daylight and iunlight antagonized each other, and thus he succeeded in obtaining the first photograph of t h e red end of the spectrum showing the ( L , ,9 and y lines below A , these lines appearing positive. “This bleaching action of the red and orange rays upon sensitive surfaces already darkened or affected by exposure is very remarkable hen taken in connection n i t h the opposite effect T$ hich can be produced on under-exposed Daguerreotype or paper prints prepared n i t h haloid salts of silver containing an excess of nitrate, by continuing the exposure under a yellow or red glasi. This continuing action of t h e less refrangible rays \vas discovered in 1840 by Edniond Recquerel, and led him to call the more refrangible ray5 rajfons cJxcitateur,, and the less refrangible taj’utis cotatinuuteuvs. It can be i h o n n quite well lvith P,O.I).,’or a n iodized sil1,er plate. It is notenwrth!? t h a t liitter, in discussing the nature of the difference betneen the action of the two ends of t h e spectrum, says t h a t water is the chemical agent in all processes of oxidation in the n e t n a y , and t h a t i t is decomposed in all proceises of oxidation or deoxidation. I n t h e first case, the oxygen combines XI ith the oxidizable body, a n d t h e hydrogen is set free or conihines with more oxygen t o form water anen-. In the second case, t h e hydrogen of t h e water combines with the oxygen of the body to form water, while t h e oxJ-gen formerly combined with the hydrogen goes t o the oxidizable body present, the deoxidation of which it usually only indirectl? brings about. I n some later
researclies he sho\ved that perfectl?. dr!. silver chloride is not acted on by 11-hite light, or the violet rays. ' ' Some -pars, ho\\-ei.er, before Ritter announced this theor!. in connection ivit 11 photographic action, our o \ v n countryivoman, ' ' the ingenious and livel!. ' * Xrs. Fulhame, as i'ount Iiumfortl calls her, had discovered, and puhlished' in i ; g ~ , . t h a tthe p x s e n c e of rvater is essential to the reduction of metals ' m o r e espcciall!- tlie salts of gold arid silver) 1)y light. ari obserx-ation Ivhich has been confirmed so far as most of our ordinar!- photographic processes are concerned. Slit. held that 1vht.n light reduces the metals, it attracts the ox?.gc.n of the ivater, ivhile the hJ-drogen of the latter unites in its nascent state to tlie oxygen of the metal and reduces i t , forming, at tlie same time, a quantity of water equal to tliat decomposed. Further, she sa!-s, water is decomposed in c\w-!- instance of oxygenation. *. Ritter' wrote an analysis of JIrs. Fulhame's book arid thus ivas \vel1 acquainted with her theories, sotlie of rrhich he seems to hcive adopted. H c himself made a good many o1)servations on the action of light anti intended to have tlc\-otetl hiinself to the study of it, hut [vas led to give most of liis cnerxies to the (lei-elopment of gal\-ariisin, This s t u d y led Iiiiii t o remark i n one of his nunic'rous papers that the result of a t1iorouF;li practical investigation ivould he that the polarit?, of chemistr!., electricit!-, tnagnetisni. heat ctc., was due t o a sirigle principle common to all. He \vas also one of the first to recognize the electrical nature of sunlight. Ilder says, however. that his hast!- and unverified electrical 11y~:ioth~'ses led him astra!.. I7xperiruents made long after Iiittcr's, l ~ yIiollert H u n t , 1 that the accelerating power of moisture in proreduction of silver salt was due to the decotnposition of the tvater and the formation of nascent h!-dropen. ScliOnhein has also shown t h a t , in all cases of oxidation, in presence of xvater, h?-d;ogen peroxide is formed. I t is well ~
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knoitn tliat this su1)stance is capahle of :tcting h t l i a s ;in oui(1i;lc.r arid a reducer, and as Sonitadt has founcl t race's o f it \\lien silver chloride is exposed to li:,.lit in itater, it is prolmble tliat it has t o be reckoned with in most casei of photo:,.raphie action. I l i a w d n e l t on these earl! researches of Nrs. 1~~11liaine and Ritter, hecauie t h e principlei the!, cnounccd a i to tlie action of moisture in tlie reduction of silver and gold from their salts b y the action of light seems t o i)t, closely connected \vith tlic phenomena of photographic reversals, R liicli it i i generally agreed are due t o oxidation, though they do not fully represent the complex actions that take place in our ordinary pliotographic proccsscs. ' * The lightening action of tlie lesi refrangi1)lc. rayi o f t lie spectrum upon darkened silver chloritic wai confirmeti iii I S r o b j S e c b c ~ k ,i t 110 also recorded somc interesting o1,sc.rvutioni on tlic tliffcrcnccs of tint taken by iilver chloride espose(1 under colored glassei, correipondinq i t ith tlie color of the t rmsparent niediuni under which it was expoie(l. \I'itli tliis aspect of the question \\e liave, lioivevcr, not liing t o do. '. 'I'hc. next procc~ssof reversal t o I)e noticetl i i oiie tliut a1)peat-i t o liave i)eeii first pu1)liilictl 1)y 1)r. Vyfe,' in A\pril, rY*?c), though several similar proceises for o1)taining positive pictures direct in tlic camera \vcre tjrought out all about tlie i a i i i c , time, in 1839 and 1840,by Bayarcl and 1,assaigne in France,, arid by Herschel, Hunt and Talbot in lingland. In Fyfe's process, paper prepared wit11 silver pliospliatci (in most of the other processes sil\Ter chloride i t as used) \aa\ first darkened b p the action of light, then ir~imer~ecl in a iolution of potassium iodide, and n hilc still moist, cxposecl to light in contact n i t h the object t o be reproduced, and left until the whole of the paper exposed became yellow.. I t thus sho\ved a positive image of the subject, (lark upon a light n trill qather around tlic sidei. If the n ater is e\ auiinctl, tlierc nil1 be found no trace o f eithcr si1vc.r or iocIi(Ic1, thus it is evident that the actioii has been corifinecl t o t l i c t pa per. \Ye see that the iodide of iilver has the powcr o f S C ~ rat inq hydrogeti from its combinations. I n thc proce5s of tl:irkcning, the liberation of hydrogen i 5 certain, hut I 1i:ivc not in an) initance heen able to detect free iodine; of coiir\c' it niust exist, tither in the d a r k e n d surface or in coni1)in:it inn with tlic unaffected under layer; possil)lj. tliis lxs I '
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the iodide of silvcr with iodine in simple mixture, which, when light a c t i no longer ?n the preparation, is liberated, combines with the hydrogen of t h a t portion of moisture which the hj-grometric nature of the paper is sure t o furnish, and as a n hydriodate again attacks the darkened surface restoring thus the iodide of silver. 'I'hii is itrikinglj- illustrative of the fading of the photograph "The picture is formed of iodide of iilver 111 it? light parts, and of oxide of silver in its shac1on.i the yellow salt darkens under the influence of light, it parts with its iodine, 11hich immediately attacks the dark oxide, and gradz ~ s of the ually cons-erts it into an iodide. The ~ ~ o d opepauzdi restoration which takes place in the dark i i not quite so apparent. It is possible that the active agent l i g h t being quiescent, the play of afinities comes undisturbed into operation -that the dark parts of the picture absorb oxygen from the atmosphere, and restore to the lighter portions the iodine it has before robbed them of." Hunt then describes a series of experiments on silver iodide which more strikingly exhibit this remarkable peculiarity. " H e repeats the above in his later tvork published in 1859, and I has-e quoted it a t length because it seems t o give a very probable exposition of the reaction5 which take place in some reversal5 and carries on the theory enunciated by Mrs. Fulhame and Kitter of photographic action being to some extent dependent on the deconipositiori of ivater preient either in the atmosphere or in the ieniitive iurfacc. "On the same principle, Poitevin proposed in 1859 a method of obtaining re\-ersed positive5 in the camera which seems t o has-e been successful in his hands for makinq stereoscopic transparenciei. ''-4sensitive plate was prepared by the n e t collodion process, with a collodion containinq lei5 iodide than usual for negatives. I t was then exposed for a f e n seconds t o direct light, the appearance of the plate Iieing unchanged. After washing t o remove the exceii of silver nitrate, it was treated with a 4 percent solution of pure potassium iodide
free from iodine, all these operation5 being performed in the dark room. The plate x ~ a ithen esposetl in the camera at)out three times longer than the ordinnrq- expoiure for negatives. It \ \ a < next n-ashetl to remox-e the iodide solution, and having been treated \\it11 a ncal, lmth of sill-er nitrate a t 2 5 percent, \vas developed u i t h pyrogallic acid, acidified with lactic acid. It ab then blackened only in the parts acted on by liqht, and after tl!e del-elopment had been carried to the usu:rl e s i c n t , the image \bas fixed with a weak solution of hyposulphite or cyanide, or a mixture of the two. The image obtained in thii manner ria5 a positive by transparence. Poitevinl gixes a list of substances uhich he found might replace the potassium iodide in 1)ririging about this reversing action---among them those rnoit actix-e were : cyanide of potassium, also when saturated ith iilver iodide; solutior, of bichromate of potash acidified with sulphuric acid; dilute hydrochloric acid. "Sir JJ-illiam .Abne? has given the ratioriale of thii process in his Ttentisc 011 P h ~ t o g r ~ z p h p. j ~ , 3 0 5 , and ha5 shonn t h a t potassium or other simple bromide can be substituted for the iodide " H e also shons t h a t if a plate prepared with silver iodide have a preliminary exposure given to it, and then be given a prolonged expoiure it1 the camera, reversal of the image will take place. If, hon-ever, such a plate, after ivashing, be treated with a iolution of a deoxidizing agent, 5uch as pyrogallic acid or potassium nitrate, the reversal TI ill not take place, nor will it if the plate be exposed in a cell containing a hydrocarbon such as benzene, or in dry hydrogen. Consequently, the presence of oxygen in some form is necessary to obtain reversal, and it is not eaiily obtained if a substance readilj- taking u p oxygen iy in contact with the silver salt. " I n 1862, Dr. Sabatier' gave a description of a method "
' Trait6 de l'imprrssion photographiclue * Phot \e\\? 6, 366 (1862)
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of obtaining direct positives in the camera. -4 glass plate was coated with a brornoiodized collodion, sensitized in a neutral silver b a t h , and given a normal exposure in the camera. The plate was developed with weak pyrogallic acid solution, acidified with glacial acetic acid. As soon as the details were out, the developer was poured off, and the plate washed. It was then treated with a 4 percent solution of silver nitrate, which was again poured off after a sufficient interval, a n d then the plate was redeveloped with the pyrogallic acid solution, the development of the positive image taking place quickly if t h e treatment with the silver solution had been prolonged-more slowly with a shorter t r e a t m u i t or a weaker silver solution. The plates were fixed in hypo or cyanide, a n d could lie toned if desired. “ T h e most interesting feature of this process is Ilr. Sabatier’s theory of t h e reversal of electrical action b y the second silver bath. He says: “\Then no more pyrogallic acid remains on t h e plate, the formation of the negative must necessarily be interrupted, and to form a color differe n t from the first with the exposed iodide not yet employed, i t suffices t o substitute for the electrical current nhich prevailed a t the first combination, an electric current of the opposite nature. ‘‘ Many substances poured in solution upon a negative, t h e formation of which has been interrupted, poiiess the power of inverting the electrical current previously tfeveloped, a n d causing the positive combination t o succeed the negative. Such are most of the alkalis, lirnenater, solution of ammonia, and especially nitratc of silver.” “ The presence of strong acids in the bath or developing solutions was prejudicial, and, therefore, preference was given t o pyrogallic acid with acetic acid which does not interfere with the transformation. “ T h e difficulty, of course, is in h o n i n g lvhen t o stop the first development, h u t Dr. Sabatier says t h a t , n i t h the method he described, successful re5ults were obtained. A l t a n y rate, the theory of tlie reversal o f current working t h e
transformation from nvq::itive to positive is of interest, tliouqli r,itlier \ ague I t 11a s fountlet1 on some observations of tlic .1111~'Ikspraty, which are gi\.en iii t h e same voluinc. & h i o t h t ~ r inethod of Snbatier's w a i by admitting light aftcr tlic (le1elopnient as started ; the liqht acting 011 tlie de\ eloped parts caused them to blacken a t the expense of the alrcncl!? partiall! developed parts Many similar methods L\ it11 \\ et collodion I\ ill be found in the journals, and are referred t o I ) \ I'der, in his . ~ z c s J i ~ ~ ? ? l l cH(i~.zcllmch.' hes * ' Returninq to the earlier periods, l i e note that shortly 'iftcr tlie introduction of the Ilaqtierreot! pe process it \ \ a s found that i f the esposurc of tlic plates in tliv camera i t a s untlul? prolonqcd, the tnercur! ir a s deposited irreg.ularl> o\-cr tlie plate. giving ;i pale 1)lue image, w i t h a partial or tot'il rcverxal. 1)r. J. \I7. 1)raper i f a s , I think, the first to stucl?. this phenomenon. a n d he attributed it to ;in x t i o r i of tlie ra! s of liqht analoqous t o tliosc of heat actirig on a mass of ice, thc temperature rising graduall!, to 32' E', \I here it s t 01)s t ill tlie molecular cliange of liquefaction i i accomplisliecl , iilicn it goes 011 rising again I n the same i v a j ? , when the r a s of liqlit bcqin to act on a Ilaguerreotype plate, the sil\ cr iotliclc commences clianqing, m t l is capable of Iwing I\ liitetictl l)y incwur!.. 'Chis proccss goes o n qratlually \\ i t l i incre~secl \ \ liiteness o n tlic more c~uposed parts until a c e r t,iin point is gaincltl, 11 lien the ioclidc apparentl!. unclergoes n o further \ isilile chanqc, but another point 1)eirig gained, i t 1 )c,qins to assume. IT lien niercuri:ili/ect a pale-hlue t itit, I)c.coiiiing clecper ant1 (keper, until at last it ass~iiiies tlic
brilliant blue of a watch-spring, thus producing the effect called solarization. " H e gave no further elucidation on physical or chemical grounds a t the time, but in a later paper' he maintained t h a t the plate was polarized t o light. He says, ' l i r e know t h a t iodine is not thrown off during exposure, b u t is abiorbed by the underlying silver, while silver is liberated on the anterior surface of the plate. From the circumstance t h a t iodine is evolved a t the back of the film and silT-er a t its front, the film itself remaining of t h e same thickness throughout, i t is obvious t h a t there is a strong resemblance between this phenomenon and t h a t of the polar decomposition of water. The electro-positive and electro-negative elements are yielded u p on opposite faces of the film, a n d its interior undergoes incessant polar changes, the opposite electrical particles sliding, as i t were, on one another.' "There is, I think, little doubt t h a t borne such action does take place; during the exposure there is a constant transference of atoms, till after a certain stage there is a n excess of silver in the most exposed parts, and the mercury is deposited more readily upon the less exposed parts, or more or less equally all oxrer t h e plate. " Draper also found the Daguerreotype plates were acted on by the red and blue rays of the spectrum, in opposite ways, similarly t o the action noticed by Ritter with silver chloride. Moser also examined the phenomenon of the solarization of Daguerreotype plates, and attributed the gradual change in the developable condition of the iodized silver surface to the action of atmospheric oxygen, which is ever present and cannot be gotten rid of. This agrees with Scholl's theory, referred t o in my lecture on the Teachings o j Daguerreotjlfie. t h a t there is a constant transference of atoms b y the continual formation of silver oxide and silver iodide, their alternate decomposition and re-formation. The w m e prin-
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ciple probably applies to gelatine dry plates, in which. however, moisture diffusing through the colloid material, no doubt plays a very active part. "During the course of recent work on I)aguerreotype, I found that plain metallic silver exposed t o light ihonetl this re\-ersed action by long exposure very distinctly when developed with mercurial vapor, and I had several instances of i t , some of which I can show you. In this case, of course, there is no halogen or electro-negative element present except the atmospheric oxygen, together with the moisture of the air. How far the change in the molecular itructure oi the film may be due to their action I am not prepared t o iay, b u t , as far as I have been able t o ascertain, metallic silver is not oxidized by exposure t o light, and anj- action would more probably be catalytic, a5 in the case of the iodide, according to Scholl's theory.' " I n 1875, during the course of some experiments in photographing the solar spectrum on dyed collodio-bromide dry plates, I constantly met with reversals in the red or blue end of the spectrum, often both simultaneously. Soticing t h a t n-hen the plates n-ere quite clear of fog the reversal at the red end did not take place, while the direct action on117 extended as far as C, and recollecting Draper's principle of previous exposure of the plate, I was able to obtain a reversed negative picture of the whole of the red spectrum up to and some little distance beyond X. In this case the lines appeared black on a clear ground, and the plates were stained blue with a mixture of malachite green anti methyl 1-iolet, known as "marine blue," b u t similar result5 were also obtained with plates stained with annatto. as me11 ai with other coloring matters and aniline dyes. "By another application of the same principle I found t h a t verj- complete rcversed negatives could be obtained 11y first exposing one of these stained dry plates for a short time t o light, and then placing it in a printing frame under I
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i n n , 68, 149 (1899)
a negatiw, over the face of \thicli ret1 glass was placed. After an exposure In the sun for a1)out twenty minutes and developing \iit11 alkaline pyro a verj fair negative ~ ‘ 1 5produced M h n expoicd under hlue qlais, hou ever, these plates gave quite a inixed result, lialf positi1.c and half negative. I t is not impoisible that suitahle gelatinc dry plates treated in t h e same ~va!- \I ould produce similar results, and such a nietliod ~vouldhave many practical uses. I arn sorry 1 have no ipcciiiiens of t h e method t o clio~tyou, but I have brought with me t n o o f tlie collodio-l)romitle p1atc.s of the spectrum, one staincd with a 1)lue (1) e and tlie other i t it11 naphthalene retl. In Alpril, 1 8 ~ 0when , pliotoqrapliinq the imi on similar 1)lue-stainc~tlcollodio-~)romideplates, I 1i:ttl a case of recurrent reversd, Init riot t o i t i full extent, of nhich an account is given in the Phot. J O Z L ~ IRlaj-, I., 1898. One of these plate5 sho\ved, with nn allnoit instaiitaneoui exposure, an unreversed itiiaqe; exposures of 5 to I O seconds ivere fully reversed, :mtl tlicn a i the c’xposures increased from 00 to S o up to z o o seconds, the reversal i t as not oht.rvet1 except just rountl t h e limb. The reversed iinaq:cs \vu-tx a l l cluite clear and slixrp. \Tit11 a n unstained plate, tlie reversetl images were tliose exposed for tlie shortest time, :ind then they gained strenqth according to thc leiigtli of esposure. \\‘it11 similar plates prcpnred \\.it11 collodion cont:tining annatto, the shortcr c.xposures ere I\ e11 rc~ver~etl, then, a s t h e espo5ure increaied, the itiiages of tlie sun nt’re not so clear, 1)ut cleared aqaiii ai 1onKi‘cr exposures from ~ o o to 2oc) seconds were given. ( ‘ In 1879, I3ennett’ itated t h a t gelatine dry plates were very readily reversed I)> over-exposure, arid showed this phenonienon of rccurrcnt rcversal verj. ckarly, the image first qaininq intcnsity up to a certain point, then losing it 1 ~ yover-expoiure, to a point \\-hen the most exposed parts are perfectly clear on developriient , then increasing in den“
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sit!. again. and tlien agaiii losing i t . Sir \\.illiaiii .ll)iie!r Iias slionm t h a t i n this exseq also, t h e tentlr~tic.!~t o rc,vcmal is dcstroj,cd 11). exposing tlic plate lxitllecl \ v i t l i ;i solutio^^ ( i f potnssiuni nitrite, \vliilc a solutioii of potassiuiii 1iicliruiii:ttc \vi11 favor i t . 111 r88o, 11. Jnnsscti cxrrictl tlic ol)scrvatiori of tlicsc. recurrent rcvcrsals inucli flirt her, a n t l , I)!proloiigetl e s p osLire , ol I t ai ne d si x d i s t in c t s t ages . I . The ordin:ir!. iiegativc) image. 2 . ?'lie first neutral state, dark grountl. ,i. T h e positive image. 4. ?'lie seconcl neutral state, clcnr groiiii(1. .j. I hc sc~otitlnegative. 6 . The tliird neutral state, (lark groutitl. I lie sccontl ncgatil-e \viis c.alciilatcSc1 t o rccittirc 1 0 0 . 0 0 0 ., times nornial exposure. 1;roin a paper by 1)rapc.r' I quote tile foll(iivitig p m graphs : ' ' '1'11 cl ;icc o 1111 )a 11y i I I g I) 11o t ogr a1 11i c i I i i 1' re ss ic )11 ( )f t 1 1c, solar spectrum, \vliicli I !vi11 t l i u n l i !'ou to give t o Sir J o l i i l 1 Icrsclic.1, \vas o1)tnined in tlic soutli o f \-irgini:i- l)rolxti)lj~ V C I U can make nothiiig like it in I, a n d the negative rays seem almost ;is clfl'ccti\-e in protcctirig, a s tlic blue ra!.s are in decomposing iodide o f silver. 'I'he most retnarkahlc part of tlie phenomenon is t list tlic same class of rays malies its appearance again l)e!.ontl the extreme lavender ray. Sir J . I-Iersclic.1 has already stated, in the case of lirornicle of silvcr, that these negutivc. rays mist low down in the spec,truin. This spcctrurn, I i o \ v "
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"
ever, proxes that they exist at both ends, and do not a t all depend on the refrangibilit). I t [vai obtained with yellon iodide of silver, Daguerre’s preparation, the time of exposure to the sun fifteen minutes. “ I n this impression, six different kinds of action ma! be distinctly traced b!- the different effects produced on the mercurial amalgam These, commencing with the most refrangible rays, may bt. enumerated as follows - rst, protecting rays; 2nd. rays t h a t whiten, Jrd, rays t h a t blacken, Ath, rays that n hiten intensely; 5th, rays t h a t lvhitcn very feebly; 6th, protecting rays. It i i obvious we could obtain negative photographi by the Daguerreotype process by absorbing all the ray5 coming from natural objects, except the red, orange, yellon-, and green, allotting a t the same time diffused daylight t o act on the plate “This constitutes a great improvement in the a r t of photography, because i t permits its application iri a negative wa? t o landscapes. In the original Frencl! plan the most luniinous rays are those that have least effect, while t h e sombre blue and violet rays produce all the action I’ictures, produced in this ~t~ y never , can imitate the order of light and \hado\\ in a colored landscape “ I f it should prove that the sunlight in tropical region4 differi intrinsically from ouri, it nould be a very interesting physical fact. There are itrong reasons to believe it i i so. The Chevalier Fredrichstal, 11ho traveled in Central .America for the Prussian government, found ver!- long exposures in the camera needful t o procure impresiiony of the ruined mmunients of the deserted cities existing there ?‘hi< was not due t o any defect in his lens; it was a French achromatic, and I tried it in this city with him before hi5 departure. The proofs which he obtained, and which he did me the favor t o show me on his return, had a very remarkable aspect. More recently, in the same country, other competent travelers have experienced like difficulties, and as I am informed failed t o get any inipressions whatever. Are ”
these difficulties due t o the antagonizing action of the tiegative rays upon the positive? ' This paper of Ilraper's brings up tito points. thc re versing action of certain ray5 and the apparent necessitj for long exposures in tropical countries The iecond part was answered by Beck' in 1904, though no reference was made in the latter's paper to the question asked bj- Draper Beck's observations are so intereiting that 'I take the liberty of quoting them even though they have no direct connection with the problem of solarization " T h e first time I set u p my cameia in Egypt I wai startled a t the way in which the picture looked on the focusing screen. Instead of a beautiful harmonious image i t looked like a harsh photograph There were crude. bright lights and immediately adjoining them, almost ~tithout intermediate steps, were deep, black s h a d o m ihowing almost no details It was clear to me at once t h a t , in spite of all theorj-, a short exposure wa5 out of the question I n fact, on account of the deep shadows, it nould he necessary t o give a longer exposure than rvould be customarj- under t h e same conditions a t home (I-ienna). .After thinking the matter over for a while I iaiz- clearly w h a t was the cauie of the great contrast5 antl especiallj of the heavy dark shadows. I perceived that the purer air of the South--just because o f i t i clearnesi antl permeah:lit>. t o light-did not scatter the 5un'i rays 50 much a i our oxtn air does. Since the shadows \\-ere lighted i o little h j diffracted light, they were necessarily heax-itr and darker than they would have been in our latitudes ' ' The phenomenon of the dark black shadows appcarecl in the most surprising n a y in the d r j , crystal-clear, deiert air of Vpper Egypt. " F o r three weeks, while I \vas partly on the Sile and partly in the desert, I had absolutely no chance t o delelop any plates, b u t I was nevertheless so conxrinced of the ac"
I
Eder's lahrhuch der Photographie, 18, I j y (1904)
c u r a q of 111) ol)ser\ ;itioii> tliat iii prcictic*dll ever! case I 11iatle longer eupostire4 tlia1i nere pertnissible tlieoreticall> Thcx results \I ere an rbiiiincntlj ,at isfactor! coniirniation o f my \ iens. In vi\\of all this’ and a s a coiisecIueiice of siniilar experiences i n A\frien, Spain, (;rewe and elien licare, I sliould suiii u p iii! eupericiices i i i regard t o lenyth of e s posure in southern countries as folloii s : Til southern countries a relati\-elj 1)rief c’xposurc i\ prinissiblc only i f l i ~ npracticall? all parts of t h e sullject to tie pliotographerl arc \\ell lightecl If larqe or iriiportant portions are in shado~f, a shorter c’sposurc \\ill tiot (lo .ind one iiiust expose at least as long as, and in very clcar, tlry air up to twice as long as, one \\oul(l do at lioinc under cciu;i11~ favorable conditions of light. Of course, tliis is after taAiiiy into account variations due to tlie difference5 i l l the season, length of d a y , and height of sun, resulting from tlie diffcsrencc bet\\ een our latitude and tliat of the southcrii couiitry. ” \\7aterhousc rcferrecl t o .ihnej ‘s I\ orL on re\ ersals, 1)ut Alliney’s paperJ is of sucli funtlanientul iinportaiicc t liat I quote it practicall! in full + * I t i. ne11 Anoiin, if ;i 1)late l)e l)reptre(l \ \ i l l 1 i l l \ er iotlidc h y the ortlirinr!. \\ c t proc 1)e I)rietl? expose(1 to light, a r i d after \\asliirig I)c trcatctl \ \ it11 a solution o f potas5ium iodide arid t lien IN exposed to a n iiiicqe in the caiiicra, t h a t , after dipping in t h e silver-bat11 and de\ clopiny, a positive iiriage is ot)tainetl. It tn:itteri not \I liether t h e potassium iodide 11e alkaline, ricwtral, or w i d , tlie ‘,aiiie effect 11 ill be rioted; also that there is no diffcrcncc~if, after trc~atriicnt with the potassitini iodide, the plate \IC x i a\lietl or not, tlie r ~ e r s a lof the iniagc vi11 \till 11c shown In tliis case tlic iodine is liberated as before, hut the action i5 increased 1)y the ztcceis of oxygen from the air; in fact it is a rnixturc of effects. If potaisiuin 1)roinidc or any siniple 1)rornitle be suhsti“
~
(‘
_I__
_ ~ _
’ Cf -1’1111
\\icticr
\IdX
f’lirit
1898, ir88o)
I3littir
[j] 10, 2 0 1
207
l ’ l i r ~ ~L i ~ r r c \ 1 i o n ( l c n / 1899 q j
tuted for the iodide, the same result ohtains. Silver iotlitlc, if prepared with an (fxcessof solulile iotlidc., or i f , after prcpiration n-ith excess of silver, it tie treated Jvitli a solul)le h o mide, is insensitivt. to lightq m t l the esp1:ination of tliis perhaps may be found in the fact alrcarl!. statcd. It has been usuall!- held that a soluble iotlidc>,sucli a b potassium, can destro!- a n invisible itnpr radiation, but this is not the case if it lie treated Ivitli tlie iodide in the dark. If, however, an!' iodide. such as cupric or ferric, be eniplo!~ed, \vhich readily lilxrates :in eclui\-alent of iodine, the destruction is acconiplished in the dark. The least favorable iodides for such destruction, as I lia\,e already shown,' are the monads. If a plate prepared with silver ioclitlc ha\-e :i prcliiiit i coiisi0inary exposure given it, and then lie exposed e ~ t r l d ct i m e to the image formed in the camera, :i reversal of the image will take place as before. If, ho\vever, sucli ;I plate, after washing, be treatetl \vith a n aqueous solution of pyrogallic acid, potassium nitrite, or an!- other ositlizing agent, such reiwsal of the image \vi11 riot lie olitaiiietl; nor will it if it be exposed in a cell containing such a siilistniice as benzene, or if ecposecl in dry h!-drogen. From tliis ive learn t h a t , t o obtain reversal, os!-gen must l)e p r e s i m t in some form or other, and that if a substance rendil!- takirig up oxygen be in contact with the silver salt, a re\-ersal caiinot be readily obtained. An intercstitig corrolioratiori of the alm\-e st:iteincnt is to be found in the treatment of an csposecl plate in ii cell containing a dilute solution o f pertnan:;.aiiatc of potash, bichromate of potash, or h\-drox!-l, Tvlicn it !vi11 1)e founcl t h a t the reversal takes place with t h e greatest facility. 'I'lie same reversals may also be obtained by using an!. o f the mineral acids in a diluted form.' "
' I
"
P h r ~ t o g r n ~ ~ lJiii~c~ i r r i a 1878. l, I t m u s t , hc~\vever. t)r r e r n e m l ~ e r c ~ tt lli i i r the - ~ I I ~ I I ~i :~i i Ii \ It I ~I w \-cry i l i l u t e , i i r tlie uliole effect o f t l i c lpreliiiiiriar>- c ~ x i i ~ i w r\ e\ i l l IIC t l c . ~ t r ~ i y e i iI i, n w these oxidizing agents :ire :ictivc i n t h e d a r k , I ) U I tiel i i i i i r ~rexilily i n tlic liqlil ''
l’hv ahove experiments slion then, t h a t a reversal may be obtained t)y the presence of the iodides or bromides (and in a more feeble nianner I have also found, b?- t h a t of the chlorides), and also b? oxidiiing agents and mineral acids, nhile the presence of a deoxidizing agent, or t h e exposure of the plate in a medium free from oxygen, prevents t h e occurrence of the phenomenon. ‘‘ IYe shall consider shortly as t o xt hether the reversing action depends upon the sensitiveness of the salt of silver obtained by the preliminary exposure, or upon t h a t of the agents employed in effecting such reversal. “TYith the bromide of silver n e have rather different phases of the phenomenon t o consider. The development can be carried out n i t h the alkaline or the ferrous oxalate developer, a mode which is more cas\- t o carry out than the development by precipitation of metallic silver from a n aqueous solution of sill er nitrate. For experimental purposei, filini containing silver bromide may be formed of collodion or of qelatine, and the behavior of the silver ialt in the t n o vehiclei is somenhat different, and has t o be considered separately Collodion is, or should be, a strictly neutral substance; that i \ , it i i merely a meciiuni in the pores of which the iilver salt is entangled and kept in position, and has no effect on the progress of development or on the action of light, he?ond that vhich ma? be due t o its physical qualities, its chemical conititution remaining unchanged. A collodion film is essrntially poroui and not continuous, as may be iecn by a tnicroicopic examination, and free access of the attnoiphere t o the iilver i i thus obtained. Gelatine, on the other hand, ii a substance readily acted upon by oxidizing agents and I>> the halogens; and consequently it may have an effect on the progress of development and on the action of light, its chernical constitution becoming altered. It is a hotnogeneous film, and not porous in the ordinary sense of the word, and is a protective agency against the atmosphere t o those silver salts which may be embedded in it I‘
“
I
" 'I'lie most convenient mcJthod of experitnenting ivitli silI-er liromide is in the form of emulsion, made either with collodion or with gclatitic, but it is not to the purpose of the present. paper to refer t o the niode of preparation beyond stating that in the former case the emulsion is usuall!. prcpared ivith an excess of sill-cr nitrate, and the latter with an excess of soluble lironiide, both of which are eliminated as far as possible by washing. "If a film containing silver tirornide, whether in gelatine or collodion, have a preliminary exposure given t o it, and then be treated with a soluble bromide of a n alkali, such as of potassium, and be again exposed t o light in the camera, it nil1 be found t h a t there is not such a rapid reversal of the image as n-ith the iodide, but that longer exposure is required to effect it. the reason being t h a t bromide of silver prepared Ivith a large excess of soluble liromide is still sensitive t o light. If, therefore, the light decomposes the soluble 1)rornitie on the plate, liberating enough bromine to form Ercsh bromide of sill-er with the suhbroniide formed bj- the preliminary esposure, that freshly formed Iironiidc, 1)eing scnsitiI-e t o light, is again reduced to the su1)brornitle state by the samc rays :yhich formed it. It i d 1 be evident, h(j\vever, that re\-ersal should take place more rapidly with the soluble broinide present than without it, and such is the case. * ' I t is useless t o treat a silver bromide film Lvith a soluble iodide, since silver iodide is inirnediatel!- formed, and t h e reactions that take place are similar to those already described. ' * If bromide of silver in collodion be esposcd to the iniage in the camera ~ritlioutthe presence of an!. other substance, a rex-ersal takes place. Iiouglily speaking, the re\-ersal takes S O I ~sixty times more exposure to the light than is requisite to produce the maximum ordinar!- effect. '1'0 trace the cause of this reversal i t is onl!. necessary t o treat the film with a 5 percent solution of potassium nitrate, rvhen it will be found t h a t the reversal does not take place. The
5aIiie Iioltls true i t 1ic.n tlie ti1111 is treated v i t l i a n y tlcc~xitliLilig solution, or if tlic plate I)e imnieried in IicsnLcnc or liydrogen. 1he cause. then, of the rc\.ersal in this case i i cvidentlJ- an oxidation, and this niay lie further 1 crified Iiy treating the f i l i ~ i after , a prclirninary e x p o ~ u r c I\ , it11 bichromate of potash, h y d r o x ~ l , etc ' ; it nil1 then be found that the reversal takes place iiiucli more rapidly tlian \\ hc1i those o u i d i h g agents \\-ere aliscnt. 'The i n m e ilia!. lie said of the mineral acids. ' ' If silver bromide 1)e held in a qelatinc film, the action of light is ioriicwhat tlifferent. If the platcx 1)e expo5ed iri tlie camera for a short time, i a y n fe\\ secontli, tlie image de\-clops in the uiual nianner and \IC ha\ e a negative image; if it be prolonged t o , sal., a minute, the ini,rg:e is reversed on development , a further exposure c a u i e s a negative iniage t c) hc. produced, 11 hilc one niucli iiiore prolonged causes a positive image again t o I)c formed on development. Here are four distinct pht.nomena' which need explanation. To solve the pro1)leni offered. plates ihould be exposed when saturated I\ itli a solution of potasiiuni nitritcl a5 before, when it will 1)e found that tlie phenomena are :il)icnt, a rew r s a l being aliiiost inipossible t o otitaiii unleis the lcngtli of expoaure lie such as to thoroughl! oxidize the nitrite at the expense of qelatine. For ordinary purposes it may be said that a reversal is non-cxiitcnt uncler thcsc conditions. ' ' If a plate be exposed in l x w e n e , I i o w \ e r (a licjuid v Iiicli doei not pernicdtc t Iirough gelatine 1 , the phenomena are still exiitcnt. If a plate t i e exposed to such an extent that there i-, n marked iniaqe apparciit hefore clevelopment, and be t1ic.n iniiiierscd in n a t e r , it ~ i i l lbe found t h a t hen the iriiage appears the gelatine refuses to \i$cll to the ~ a n i e extent t h a t it does wlien t h e liqht 113s not acted. Taking these t n o experiments together, it is e\ ident that the gclatine has played ionie part n i t h the silver tiroinidc. It may therefore be presumed t h a t the three last phenomena are due, t,
the 1st t o the oxiclation of t h c iurface-particles of the lx-oiiiitle ant1 a conicquent change in color, the 2nd to the climge in color of these particles 1)erinitting the colorecl rays to TI tiicli it i i wnsitiIe to strike a deeper layer, :md t h e 3rd to the oxidatioii of t l i i i la!-er at t h e espcwse of t h e gclatirie ?'lie, - the developer than is the normal salt. On further illumination, especially in prc=ieri.ce of oxidizing agents, the suhl~roniidc takes u p oxyxen and forms an osytiromide of t h e g:t>neraltype A4g,,l13r,tOx, i n ivhich t h e silver molecule is sat urnted again ant1 thereEorc less readil!. recluci1)le. Tlic consequence of this is that a n ol-er-illumitiatetl sill-er 1)romidc is less rc:itlil!. blackened that1 one that has recei\-wl ;i niodcrate c.sposure. This theor!. ivas p u t forn-arc1 lj!, Almt.!. and furthcr extentled 1 Luggin. I ' .lccording t o Ir,is not constan1 for all kintls ( i f Illxtcs. I
''
bot 11 silver 1)roiiiide and silx-er iodide solari/e sotiien liat less
readily than pure sil\-er t)rc,tnitlc gelatine plates. ' ' The develo!ment is not ivithout effect on the plienoiiienon of solarization The stronger the developer and the longer one develops, the more readily the plate solarixs I n many cases it is a question of the developer n-hether the plate \\*it11 \-cry dilute developers solarization solarize5 or not phcnoniena are much lei4 frequent than TT it h the rapid developers. -In oxalate developer containing :i good deal of potassium lironiide eliminates t h e solarization phenomena to a greater extent than the wnie c ~ e v e l o p cwithout ~ restrainer. On the other hand, addition of sodium hypo\ulphite to t h e ferrous oxalate developer increases the solarization effect IT it11 o\.er-cLxposed silver bromide gelatine plates ; the solarized, very transparent, portions of such plates have a tlifferent color { reddish) from the non-solari7cd (blackiih i portions. [Eder]. "Metol with a n cxcess of animonia, ( r amidol ivith ammonia, arc a p t to give inveriions :inalogous t o so'ariz ition i t itli silver brotnde gelatine.' -1 similar effect i i ohtainetl n i t h a dilute nietol developer containing a qood deal of alkali and some thiocarbamide' or v itli an eikonogen de\ e1opc.r containing thiocarbamide, allyl thiocarharnidc, or especially still better phenyl t11ioc:trliatnide In fact. lT7aterhouic macle this a method of preparing poiitives d i r e c t l ~. Starting 11ith an ox er-exposed silver iiroinitle gelatine plate, which ~vould shon. marked solarization phenomena when developed in the ordinary i t a!-, a normal dc\.elopnicnt can be obtained b j treatinq the plate for fifteen minutes with a solution of ioo cc. water, =, cc sodium h ~ ~ ~ o s u l p h i t e solution (I : 2 0 ) and 3 cc dilute sulphuric acid (,i . ioo), washing, and then developing.' The solarized silver lirornide is presumably converted into the orthodox developable silver bromide. The result is very uncertain. "
’‘ Solarization plie~no~rnenaoccur alio ivlietn tlie silver ~ ~ even \vhen bromide image is de~elopecl p h ~ ~ s i c a linl ~fact developed after being fixed as Sterryl has shown. Sterr!- considers that there are two latent images, one organic and the other inorganic; a n t 1 that these l x h a v e very differently under certain circumstances.? * ‘ Soon after tlie introduction of the collodion process it was noticed that two distinct latent images 7.vere impressed upon a sensiti\-e film a t t h e same time ti!. t h e action of light, but that iv-hich is found to remain after fixing the plate has received comparatil-el! little attention. “ T h e late l l r . I--two hours' soaking) destroys solarization completely, so the strips, "
1
l . i i i ) i ) [ i ~ ~ r ~ i i i i ~I,lii-i .d
,
16, 48 I
( i c j i i z 1.
I t \ v a s i i r i i v u l t i i : i t i1ri1niitii. I \ ~ I \ sct t'rvc Sch:iuii~ EcIcr'\ ~1:ilirIniclitlcr 11 '1 ipliie, 18, 7 0
(Iello~tcolor \t liich fresh 5ilver iodide '1cyuirt.s in tlayliylit. The iodine, \\hich h,,d been previously set free, recombines under the influelice of daJ-light. Regeneration takes place in ;i fell wcontls if iilver iodide, blackened 1 1 ~cathode ray\, i i 1)rouqlit into the field 11 liere the discliarye of positive l i q l i t can pas' throuqli tlic iodine vapor If the w i t i i to lie rr.yenerated, it must ,ilw 1x2 heated ;I little. " If one starts ~ t i t l iyellon sil\ er iodide in the part of the tube tlirough \\ hie11 tlie p s i t i \ - e liylit p a i i c . 5 :ind if, after the tubc i i exhausted I er\ tliorouqlil! , a little silver iodide is decomposed b ~ t -h e cathode ra!.,, tlie \ill er iodide exposed t o tlie positil-e light t a k e \ 011 a n ormye t o r e d color. This is the color \I hich also appear> if iodine vaporize'i in n closed \pace near yellon iilver ioditlc. llic colorinq i i duc ,ulely to a p l i ~ iical :itliorption of tlic iodine 0 1 i e ipecial factor come5 i i i . lio\te\ er, in the production o f tlic color liy tiiems of the clischar::e, as ran lie i e e ~ iif one iprendi the ~-t.ll(~il iily e r iodide over tlie \thole lc~ngtliof tulle like that in Fiq. 3 . Iri the middle of t h e t u k the catliode raJTs, i t a r t i n q from K , set free iotiie iodine I n R short time the \il\ er iodide t)ccuiiie, colored a deep orange o\-er the stretch K A l itliere it ii C Y posed t o tlie positive liylit The salt in the left half of tlic tubc rctiiains unclimgeti 111 color and only after a lonq period of cliicharge doc5 the left iide aisui~iea color XIhicli even then is much fainter than that of the right half aiitl rrhich corresponds t o the ordinar! adsorption. * ' If 11011 a part of the liiylil! colored riqht-half i i wrapped in black paper \Thile t h e rc.st is exposed to daylight, the orange "
"
r i
the exposure hac1 1 ~ x 1 1 several million t itnes too long. Ti7eisz1 found that thc. solarization was eliminated t o the same extent regardless of Tvhether the plate \vas transferred direct from the chromic acid bath to the de\-eloper or whether it was washed for fifteen hours in running il-ater. Of course if the chromic acid solution is too concentrated, the normal latent image is also destroyed, as has been shown h!- Sheppard and blees,' who say : " It has frequently been maintained t h a t halogenizing and oxidizing agents do not actually destroy the latent image, b u t only retard development. Sterry,? for example, considered that their efi'ect \vas t o delay \\-hat lie termed secondary de\-eloptnent," I'. (>., an assumed intensification of a primarily formed image silver from neighboring granules. Our experiments Jvith chromic acid suhsequent t o exposure led t o the follov-ing conclusions : Plates were clipped b y rotation in CrO,,solution for a given time, then rinsed by rotation and ~ l c \ - e l o p e d . ~ I he plate-curve was distorted at the top, but log i ant1 T~ were unchanged." The velocity constant K of d e ~ e l o p m e n t vas diminished, approximatelj- in proportion to the Iognritlzvz of the strength of the preliminar~. CrO, bath. I'rolonged washing never entirely annulled the effect , but it \vas diminished. The values of K ithe velocity of de\-elopinent;~decreased with the time of inirnersion in CrO:,, ultimately reaching a iiiiniiiium for each concentration, t h c function of the effect on k' lieing independent of the concentration. ( ' From these facts \ye concluck that the CrO,, is irrcLwsit)ly absorbed in the film, prol-)ably both t o t h e film ant1 to 'I
.I
I
%?it 1il1ys Clict!i , 54, ,\.I I f I ijoh I Ilroc. liily S K .78.4, . .+(:? ( i i p b I I 5Lvri-J.: I'lirii J ( I % , I I -, 54, 50 ( I I ) O ~ I
For t h e ~ l ~ l i l ~ e'i ! 1wri.ilieiital ~ ' ~ itiFtiioclsposuresu p to 2 or 4, -4ftc.r tliree to li\.e iiiiiiutes' actioti the solarizutioti itnagel is oril!~ pm-tially tlehtro!-etl ; 1)ut it is cliaiiiyd ver!. iiiueli b y a n attack lasting fifteen tiiiliutcs, iiiucli niore r l i a i i ilic iior.mal iiiiage, 50 tlint tlic. filii1 aEter\vartl. tle\.clops almost entirely as a nornial ininge \vlictlier tle\-elopetl cliciiiic:ill!~ or pli~-sicdly. C'oiicentrate(1 nitric acitl lriiigs a1)out tliis change morel rapicll!- ant1 tiiorc' c'f~cc~tivc~l!.. ''I . . ,I hese espeririicnth also iurnisli t lie proof tliut tlic visihltl lilacketiitig of tlic silver 1)roiiiitle stancl> in no causal relation tvitli t h e fortiintion of t lie sulistniicc cciniposing the, solarized itnngc. 111 sciiie ' . tl1c t \ Y O plleiioniena see111 t o run parallel; b u t , \\it11 silver 1)roiiiitle eoritaiiiiiig hilx-er nitrate, visible 1)lac.keiiiiig takes place loii:,. 1)eforc any solarization image is forriied. Ii7itli pure si1x.i.r lm>iiiide collotliu~i i d
-
, ,
60
exposure is 3ooo-5ooo times the unit exposure, thus much longer than is necessary lvith a pure silver bromide collodion exposed under water. The traces of silver nitrate, ivhicli cling t o the silver bromide even after long n-ashing \vith water, cause a very quick and dense blackening of the plate, so t h a t the solarization is quite masked by it. .A short treatment of such a plate Ivitli nitric acid isp. gr. 1 . 2 0 ) after the exposure and before the development changes the plate to such a n extent that solarization can readily be detected when the exposure is 1000 times the induction value. The solarization image, however, will not stand up long against nitric acid (especially a strong acid); but it is gradually destroyed so that finally a quite clear, pure negative is ohtained either by physical or by chemical development. ' ' I If one exposes a silver 1~romidecollodion plate under a scale pho'tonieter for different lengths of time from thc limiting minimum rvay up into the region of solarization and of visible blackening, up to 3000 t o io000 times the norriial exposure; and if the plate is then fixed with thiosulphate and afterwards developed physicall!., ~ v efind : I . On the portions ivhich have received b u t little light, scarcely more than the induction limit, there appears a normal negative (kiega2icc. o) the first order). The substance forming the latent image withstands to a certain extent treatment with nitric acid before the development or a fixing with thiosulphate and a subsequent treatment lvith nitric acid. '(2 . \'l'ith strong over-exposure we get the solarization image, a positive. The limiting illumination at \vhich this appears is undoubtedly displaced by the preliminary fixing and subsequent physical development, but the approximate point is easy t o recognize, and the silver bromide is also I'
( '
' I iiswtiic rli:it i t is \\ell I < n i ~ \ v i i~ l i , ! r l l i v s i l l a r i m t i i ~ i iliiiiit iilr ex1itIsvd silver brc1inide I>I:itcs c a n 1)c forcer1 liark very ft!r liy [lie si]-called ret:irtlcd develiipiiient (,ilk:iliric l)yr.sical development than they image resulting from normal illumination ; there is thus dvfinite solarization. AAiciLieous chromic acid tlestroj-s both tlic solarizetl and the normal latent image to about the same estent' rdifferent from silT-er bromide). Both forms itlie tiornial as \vel1 :I.; the solarized latelit image, resist ;I thiosulpliate solution at least to thc extent that they afterit-artls respond to I)h!-sical devc1q)nient. In spitc of this the tliiosulpliat e has actuall!. cntised a n important chcinical clia~ig:c. i r i t lie nature of the latent image. Originall!. tlle normal latent image, rcsulting from a sufficient exposure, \vitlistootl tlic action of nitric acitl ver?. ii.ell. .After t h e prelirriinnr?; iising \\.it11 thiosulp11ate solution, tlie iinagc i y coinpletely tlcstro!-ecl 1 ) ~ . nitric acid [si). gr. I ,101actiiig for onl!. five minutes. O n t h y otlier hand, the latent so/izi.izczti'oii image on sill-er iodide, after fixing \vitli t Iiiosulphate, resists nitric acicl for a short tinic t o such an extent that a solarized image ma>-aftern-ards l)e de\-elopecl ph~-sic:iIl>-, If the nitric acitl acts for a longer time. lion-ever, tliere is such a charlgc that :i normal negatil-c del-elops instead of t h e solarization image. 'I'hcw reactions make it protiable that the latent , developable irnagc. on silver iodide collodion consists of sill.er subiodide ( (
" ,
"l'lic. ,III(I tlic I I
Iigiitt.. ~ I I ~ I Koi'
f < ( l c , I'lioir
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;I
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tiit.
1liii1~111~111,1tc
~ l i c( I ~ \ ~ : I I I I L >sIi-~ l ~ . c ~ i ~ u e i i tII.c.(I ly
~ ~ ~ ~ r r ~ ~ 1902, s ~ ~(145 ~ ~ t i ~ I ~ ~ i / ,
\f-liicli is tlecoiiiposed bj- tliiosulpliate allnost completely into silver iodide a t i t 1 ~netnllicsilver 1 diffcrcrit from tlie 1 ~ havior of the li!-potlietical sill-t,r su1)l)roniide in the latent image/. Explanation of t h e recorded phenomena i n t e r m s of t h e subhalide t h e o r y The sublialitlc theory is able to explain \vitliout tlifiiculty the complicated csperinicntal hehavior, toward chemical reagents, of tlic latent image in a silver 1)romide film. 7'hc' . C 7 d h $ f t i > ! ~ . ( ' fhc' ? ! ( ) ? . ) ) l ( ? / / ( ? f c 7 1 i i 2'ItltlgE :11Cgat2'iC ( 1 ) the; f i r s j t o i d c ~ . ' )consists of silvcr hroniide reduced in \-arying amounts t o sti1)broinitle. '. assume furtlier for tlie cai;e of progressive illumination t h a t , in consequence of a pliotocliemical setting free of bromine, at tlie begiiiniTig of tlie exposure to light tliere is found a sil\-er hromide correspoiiding \-cry closel!- in coniposition to normal silver I)roiiiide. 'I'liis sul)lxoniide, liorvever, as ~ v esee froin tliv pro for making photographic negatives is m o r e rapitllj- reduced l)!. reducing agents to nietallic silver than is tlie pure unexposed silver bromide. This modification of suljhroniitle is tlcstro!-c.tl hy thiosulphatc as well as by nitric acid. 1 - 3 ~means of this assumption we explain all t h e phenoiiiena rccortletl in regard to the latent silver liromidc images \vhich result from 1w-j. short exposures. ' ' \\-it11 a sonic\vliat longer csposure there is formed a silver subbromidc whicli constitutc,s the latent iinagc of tlic normal negative. It is onl!- diglitly attacked b>- nitric acid. \17ith increasing cbsposurc. it !.ieltls on tlevcllopment a riornial riegat i1.e \vit 11 t lie tioriiial cliaracteristic blackening curves corresponding t o i t . 'I'liis \il\-er txotiiide is less readily solublc t h a n silver broiiiitlc i n solutions of the fixing media, such as aniinoiiia, miiiioriiuin hromitle or thiosulphate. =Ifter tlic preliniinary fixing of the latent image this subbromide thcrefore remains as a residual image which can he tie\-eloped physically in spite of its having undergone certain changes. L)epentling 011 the cheniical nature, the ''
"
concentration, or tlie tr.iiiperature of tlic iixitig 1)ath. this sill-er subhromide is developed more or less complcately into the soluble sill-er 1)romitlc and untlissols-et1 metallic silver so that the resit1u:il image consists of su1)lJrornitle mixed ivitli more or less of metallic sil\-er. I'otassiun: cyanide c a u s e i the greatest decomposition arid ivit h it the residue consists of metallic silver only. * ' If one allom-s nitric acid to act 011 the latent itn:ige after it has heen fixed, tlie substance foi-iiiiiig the ininge is destroyed still more. the destruction being greatc.r t he more completel!- the image consists of metallic silver. Since the silver is dissollwl, more or 1 of the image 11-ill bc re1novet1 and there will remain lwliinc-1 some sul)broiiiicle, tlie presence of ~vhich can bc shon-n I)!. ph!-sical development. Hut even this is broken tlon.ii, tiy :i continuous treatment \vith concentrated nitric acid, into silver \vliich dissolves and sill-er bromide which does not respond in the dark t o pliysical development, but ivhich at once forms new nuclei for physical del-elopnient i f it is exposed to light antl this converted again into a de\-elopahle subhroniicie. ' * \T7ith a w r j - strong ol-er-exposure of the silver bromide there is formed a : o l ( ~ i ~ l ' z ~ z f2l ' )o~~u~g c ,the suhstance of \vhich is not identical irith the tlirectl!. 1)lacE;enetlsill-er brornide \vliich is apt to 1)e foririetl a i t h e s:iiiic time. 1 his substance has a tliffercnt clieniical composition and different clieriiical propertics from t hc, s u h t a n c c forming t h c negative iniage o f t lie first order. ' ' \\Tith more intense over-expowre antl sulxecluent dc,vclopinent n-e get tlie negative of t h e second order. l'lie latent image from which this \vas formed prol)a1)1!7 consisted of a mixture of at least three clieriiically different substanccs. namely of metallic siIl-er* cif the sulistance constituting tlie solarization image, antl of the suhbroniide cmstituting tlic image which j-ields the negative of the first order. .ill these substances are shown to 1~ present riot only 1 ) ~ .their photographic behavior ton-ards the photographic del-eloper, but I)? their different chcmienl 1)ch:ivior to\varcls thiosulphatc,
..
aminonia, nitric acid, etc. The iubstance formed h\- the direct blackening of 4 s - e r 1)romitle in light she\\ s a different photographic action froin that of the latent or the solarized image. This can be explained less ne11 011 the assumption of a solid solution of variabk anlounts of .ig2Rr in silver brwnitie. It is more probable that different nioclifications of sils er subhrornide occur in t h e tlifferentl? illuminated portions of the silver bromide film. “ T h e latent image on silx-er iodide \Fit11 an excess of silver nitrate seems also to consist of silver 5ubbromide, hich, however, seems to lie more clasily decomposed into metallic silver and silver halide than is the case u i t h 511s-er 5uh1)roniicle. ‘’ The substance forming the normal latent image on silver 1)romicle does not var>- in itz qualitative behavior totvards chemical reaqents 1% it11 tlie \vave-lengths of the liyht t h a t produced it. Color sensiti/ers eosine, ethyl \-iolct, etc.), which mahe silver broinide light-sensitive to light of longer TI ave-length, forcca the iilk er bromide to the m n e photochemical reaction of the silver iuhhromide formatior. in the production of the latent iinage \I hich is characteristic of silver bromide 1)) itself in the blue, violet and ultraviolet regions of t h c spectrum Xhne\ believed that the forcing-hack of the iolari7ation, 11 hen potaisiuni nitrite iz added to the film before C Y posure, is clue to the reducing action of the potassium nitrite. Luppo-Cramer’ maintains, lion ever, that the only action FF hich nitrites, sulphite5 and silver nitrate have in co~iiinorl is that of acceleratinq the 5plitting off of lironiine from tlie silver bromide Being “ chemical ” iensiti/ers the>- increase the light-serisiti\.ene~sof the plate, hut Luppo-Cranier feels t h a t me do not knon at all 1\11! they prevent solari/ation. \\7ilbert- made e.;pc.riincnti with sils e r lironiide plates wliich 11ere expoied lie! ond the solariLation limit. \Yitli ~ under a tleiise a n exposure of onc minute to diffused d s light ”
’
I’liiit I,tiii
L ~ i r i c ~ l ~ ~ n1904, d ~ ~( i~ / i
. 1.
Iirliiicli t i
r
l ~ l i i ~ t ~ ~ ~ r 1~7 ~ , i l i i ( c1 0 I ~ J
dinpositi\-c, there \ v a \ partial ,soliirization ; \\,it11 exposures (if froiii four t o eight minutes tlieril \vas cotiiplctc~solarization \\.it11 1)ut slight differences tlue t o tlie length oi esposure. I n t hc: camera a11 esposurc o f one-li:ili secoiitl ga\-e a noriiial negative; tvitli a larger stop :ind II longer exposure, eighty times tlie amount of light still gnx-e a negative, though :i Hat one, when the! plate ~ t - a s tlevelopecl lvitli the same tiietolli>.tlrocluinone cles-eloper. \l7itIi eight times ;is much light tlic plate \vas in tlie so-called zci-o state, 11c2ing pretty thoroughl!. fogged. h u t slioiviiig signs of reI-ersal. \\'ith ten thousand times the amount of light, ;I conipletel!- reversed, solarized image \vas ol)tainctl. tlie developnieiit taking placc. it1 tlit dark m t l lasting three minutes. A - over-exposure or some other ahnormal action of light, tvhereas to produce these new re.i-ersals even less t hari the ordinary exposure \vas sutiicient, and the>- appeared to be entircly due to some peculiar action of the thiocarbamide added in very minute quantities to an alkaline eikonogen developer. "-It that time I was quite unable to offer a n y opinion as to the probable cause of these reversals. or as to how they were produced, beyond stating thc protiability t h a t , although there were many points of difference, the>- \vould be found to be in accordance witli tht. generally accepted theor>-worked out by Capt. Abney, and w r e due more or less to oxidation or rchalogenization of the exposed parts of the filtn, a n d t h a t owing to the peculiar reducing action of the alkaline thiocarbamides the film during development was practically in the same state as if it were oI-er-exposed. I t seemed also probable that sulphur was the active agetit in producing r eve r s a1s . * . Further work with these curious salts and especially r-(;en.>r;il \\.; i v i - l i i i i i s e I\ :< I L O sen4 s-vsrlil l)L1lm-s iri l i ' i ! (111 sol:iri/:lti~iii, t 11;1vc. I)cen ( , i grc:il :issi's experiments have proved conclusively that solarization can be checked by the presence ciuring the exposure of certain reducing agenti. such as potassium nitrite, and can he increased by the presence of oxidizing agents. I t seems therefore quite possible t o simplify '
Intcrn,it
I < i i n q r c \ ~c t ~ i q c ~ inl i c i i i i c Ilerlin 4, i
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our conceptions and to consider that solarization take\ place c. the action entirely in the time t z j t c i the ~ ~ z p u ~ u i ll7ien of tlie light has ceased, tlie 1,roniine set free can convert t lie photobromide back into nornial hroniide \vhich is niorc tliflicultly reducible and therefore causes the phenomenon of solari7ation. Of course such a conception has no other value than that of a 11orking hj-pothesis ' ' This conception h a i the special advantage of makinq t he lesser effect of intermittent illumination intelligible. l o test it more fully I have extended to silver bromide gelatine the experiments of Xbney which rcferred especially t o collodion emulsions. These experiments are cloiely allied t o the earlier experinienti on the use of acetone bisulphite in a retarded developcr. ;\part froni the fact that the acetone 1)isulphite. like an?. acid iulphitc, decreascs the reduction velocitj- by neutralizing partially the alkali of the de\-eloper, there are other reactions 11 hich are prohablj- the itiiportant ones. I ha\-(. prex-iousl?. called attention to the fact that acetone bisulphitc reacts TI ith the oxidation products of tlie de\-eloper and therefore acts as a reducing agent. 1 his is confirmed b j a nuintier of experinients in which plates itere \)atlied in a 1-5 percent solution of acetone bisulphite, dried, and then exposed t o intense light ll\'hen these plates are developed in a normal de\-eloping solution. it is seen that tlic amount of solari7ation is iurprisingly small Even the i u n ' i image is onlj. reversed mith dif5culty. .It the \ame time anotlicr change h c o n i e i evident, t lie formation 1)j. tanning of large grains \\hen tlie amount of acetone hi\~ilphitei i hiqh This tanning action is also one to which I have previously calletl attention I t decreases the diffusion i n the film arid I look upon this as t h e essential reason for the advantages due to this compound in the c a i e of a retardetl (leveloper, though of course all three ca~isesn-ork together. "-Ilong tinir ago Alhnej.' called attention t o the fact that stronglj- illuminated portions of ordinary silver bromide '1
r ,
"I re'it1i.e
~bii
1 ) 1 1 ~ 1 1 ~ ~ g r 1893, ~ i ~ i I i309 ~
s\vell l c i s i-ca(ltlj tli,iii tlie ot1ic.r portions. 'l'liis i i . t I \ \ a j s very noticeable if one la! i in \\.iter a plate \rhicli hai received a 1011:. exposure to tlie i u n . '' It appears to 1)c iiiiportant to cstat)lisli that no solar izatiori can occur if the d c \ elolxr 15 alrealtlj present in tlie film at the time of exposure '1'0 carrj- out t h e eym-iiiienti thc plates were saturated Lvitli :i de\ eloper iolution containing no alkali, and after\\artli clried. 1 he! \\ere then e\posed to any briyht source of liqlit for :i iuita1)le time ilony enough so t h a t ith ordniarj plates ant1 ordinary de\ eloper, ould I)e vcq- IiiarLed, and \\ ere tieveloped solarization simply 1 ) ~ -being laid in a soda solution.' S o solarization can take place so long a i thcrt 1~1 any unchaiiqetl reducing agent on the high lights. 'l'lic. followmy experiment, for instance, shon s h o \ v far t h i i lioltli . On an ordinary plate, lay one iiiipreqnatetl 11 it11 dcveloper, a n d on the film of the latter la) a piece of metal ]\it11 holes cut in it Iiupose to full wnliglit for five iiiiriutci and then develop. The itnpregnated plate she\\ i a i ordinary iiegati\-e TI hile tlie xiormal plate is coiiipl I \ iolari7ccl even tliougli the light falliiig on it has passed tlirouqli the iinpreynated plate.2 This result is ciitirelj in .tccortl \\ itli .\l)nc! '5 c.uperiments and with tlie tlieoretical idc'ii 1 Iiai e outlined '1'1ic-x are, Iio\\ cver, a nunil)er of iiec r j coiiiequencei hicli iiiuit still be t es t et1. ' ' !ible to prepare the t n o constituents of the latent imaqe separatelv bt purely chemical methods rr ithout the inter\ention of light. If an unexposed dry plate is treated n i t h 1 ery dilute bromine n a t e r , there is formed a latent perbrornide image which develops in an indoxyl bath to a pure blue ‘indigo image ’ On the other hand, if one treats an unexposed plate with a very dilute stannoui chloride solution, there is formed a ‘ latent subbromide image ’ I\ hich develops in the indox!l bath to a black ‘5ilver image.’ -is ;i result of these experiments I adopted the \-ien that tlie latent image is formed according t o the folloir ing equation : e ,
+
;&lg]Br=--1qRr- - .lg2Br
’rile film of a iiller brornide gelatine dry plate, after expo5ure to light, consists therefore of the follon ing three substances “I Silver perbromide, AlgRr2 “ 2 Sill-er subbromide, _iq,Rr or -lg A XgHr. “3. The original unchanged 5ilver bromide, .lqI3r. On the basis of 1 hesc results and supported by further experimental d a t a , I attempted finally t o give an explatiaOn a silver brotion of the phenomenon of solarization niide gelatine dry plat^ which had been over-exposed t o the point of iolarization, I Itas able t o \lion. that the ’indigo image‘ developed by means of indoxtl is not solarized and t h a t , if the plate is first fixed and then developed phvsically, ”
1vitIi a potassium cy:inicle solution, the sill-c,r iiiiage (lissol\-e.;, as I lial-e s h o n ~ ibefore, ant1 t liere no\r reniaiii,5 o i i l ~ . t lie pure' 11l~ienegatiiy indigo picturc. l ' l i i b htateinent I h a ~ ysince found is onl!- partiall!, accurate :inti recluii-c..; tlic liinitatioii that the plate .;hall t i e tlcveloped i r i the intlos!-l hat h only long eriougli t o produce a tiegatii-e \\,it11 plent!. of (letail, in other n-ords for about 6 t o I O niiriutvs a t a h n i t 20' C . If one leai-es rlie p h t c in the indox>-l 1)atli for Tie;-Iiay)s h a f an hour to :in hour, the picture gxins coIitinuall!in strength a n d finall!. l)ec(.~mesa dark grecniih-l)lacli 1 i oiie tlien fixes tlie platc and puts it in the potat.siiini c!-anitlt. Imtli, t h e indigo image remain5 lwhind as ;I 1)eautiful s o / ( i ~ iz(itjoii iicgtrti':,~'. 'I'he 5cale fields froni I to 1 0 :ippe:ir t o l)e ~olarizetl,1\-hile tlie otliers, up to about 2 2 , conic o u t :i\ ;I I )I IIC' in (1igo nega t i1.e ivi t 11 normal gra ti a t ions. It \vas not difficult t o find an explanation for t l i i > , / ~ p t i i . ( , i i tsoltri.iaciti'oiz ; it turned o u t that finel!. di\-itletl >il\.c,r i n I: potassium c!-anitlc, solution u-as able t o retlucc intligo, i'. t ' , , to dissoli-e it a s indigo-irhite. If oncx place.; finel!--cli\-idetl iii(1iKo :ind molecular sill-er in a test-tuhe, acltls :I t \vent!p~'rc\eiit potassium c!-aniclv solution, a n d heats 4iglitl!., ;rftvr :i while a portion of t h e indigo goes into .mlutioii ;I; iii(Iiyi-ri-liite. If tlie wlutiorr is no\\.filtervtl. ;i clrtnr. lixlit \.~'llo\i-solution is o1)tained from \rliicli air cluickl!. preci1)itL1tc.s clot.; of intligo ;igain. I he saiiie proce\q ol)i.iou.;l>. I akc.; place ~ v l i e n n n e treats \vith pot assiuni c!.anitlv iolu t ion :i negative \I-liich has heen dcvrtloped niuch too long i i i "
,
I
inclox!.l Iiatli. The ' si1l-w imagc' dissolves thc incligo-. itii:ige,' \\%xi thcc amount of sil\-er is the grc:itest tlie krction lasts the longest, i'. c.. a t t h e points ~vlicretlic i1luiiiiri;i . tion lias been strongest On the other harid, if otic fixt.5 a n d i~xslivs;I platy T\-Iiicli has I)een tlevcloped for ail excessive length of tiriie i i i ;in intlos!. 1)ath. :ind tlien puts tliv ~)latc,i i i t o :i potassiitiii c!-nnitlv h t h to ivhicli :i little potxsiuni fcrric!.ani(le 1i:is this oxidizirig ngcnt contracts the rcduinga ~ \ t i o n of tlic silver. I tic. ' sill-cr image ' ' clissoll-c.s \-et-!. rapi(Il!, :in
'
,.
and there is left a nun-iolariyed indigo neqatiL-e with normal gradations. If this trcatmcnt is qiven to these plates which have heen c~xposedfor an equal length hut tv-hicli have been developc~lin the same indoxyl hath, one for ten rninutes, another for thirty minute\, and tlie third for an hour, the result is three indigo negati\-es of exactl?- tlie sanie itrength and gradation. From this it follows t h a t t h e prolonged action of the indoxyl developer iiiteniifies the silver image only and not the indigo image. This is perfectly intelligible in view of what has been said in regard t o the nature of the latent image atid the process of development.” I n Part 11, I i i t e n d to take up tlie work of Nipher, IYood and Lugzin. There is also an i i.portsnt paper by LLilppoCranier to be considered, t o say notliinq of the recent work of Trivelli. After that I hope to slmw th: uiefulneis of the theory of Grotthuss.
