Topics in...
Chemical Instrumentation Mifed by S. Z. LEWIN. New York University, New York 3, N. Y.
These art&, most of which are to c o a t e b d by guesl aulhms, are intended to serue the readers of this JOURNALby calling aifenth lo new developments in the theory, &&n, or availability of chemical laboratmy instrummlafwn, or by presenting useful insights and ezplanationa of topicd lhot are of practical importonce lo those who use, or leach the use of, modern inslrummtotion and instrumatal techniques.
glass, so-called after the lead mining town of Flint in Wales, contains sufficient lead to give it a much higher refractive index Lhan crown glass, as well as a faster rate of change of the index with wavelength. In the visible region of the spectrum, t.he dispersion of flint glass is about twice that of cruwn glass. Thus, a flint glass prism of a given angle spreads out the component wavelengths of white light over twice the distance of a crown glass prism of t,he same angle, as i1h1strale.d in Figure 19.
XXIV. Instrumentation for Micrometry and Microscopy-Part
Two
S. Z. Lewin, Deportment of Chemistry, New York University, New York 3, N . Y.
Dispersion The rclrnctive index of the t m s p a r e l ~ l . ~nedium constituting a lens is ditferenl for each individud wavelength of light: this is the property known as dispersion oJ wjraelim. To "disperse" means Lu spread or separate, and dispersion phenomena are manifested whenever n nledium interacts with a disturbance in u manuer that is not independent of the energy, wavelength, or other characteristic parameter of the latter. This specitieity of interaction can serve to &ifferentiate the individual energies, wnve-
Figure 18. Comporiron of the dispersion of refroetion of cmwn glarr and of flint glass over the visible region of the spectrum.
lengths, etc., and hence to sort them oul, or "disperse" them. The spreading-oul effect implied in the dispersion of refraclion is clearly evident, e.g., in the separation in space of the paths followed by rays uf different wavelengths when a singlt. beam containing those wavelengths is it)vident ona. prism. The rate of change of the refractive index as a function of the wavelength is quite specific and characteristic of the medium. Figure 18 shows the refractive dispersions of crown and flint glass, the most commonly employed types of opti~.al glass. Crown glass is a potassium ral& x n silicate gbss with refractive index i l l the vicinity of 1.50; the name comes from the crown-shaped drop this glsss forms : ~ the t end of a blowpipe during one phase of the manufacturing process. If some or all of the calcium is replaced by barium it, the glass formulation, the produrt is called a barium crowu glass. Flinl.
Figure 19. Equol prisms of flint and crown gloss yield spectra the rpramdr of which are in the ratioof ? : I .
Chromatic Aberration If we consider s lens of orown glass with a suitable aperture that will yield a ~.easonablygood focus for light of a given wavelength, as shown, e.g., in Figure 11, the fact that its refractive index is diferent for other wavelengths means that, it will not bring rays of these other wavelengths to the same image point. Since the refractive index increases with d e creasing wavelength, the short wavelength rays will be brought to s. focus closer to the lens than will the longer wavelength rays. This is illustrated in Figure 20.
Figure 20. Chromatic aberration arise. from the fact that the shorter wwelength rays ere brought to 0 focus closer to the lens than are the longer wavelength rays. Screens placed at AA and BB show spoh that are violet fringed in red, end red fringed in violet, respectively.
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Chromatic Correction Any two dilTerent aavelenglhs ran he hnlught la a forus at Ihessmeimagepoint by n sui1:hle ri,mlrimlian ,of a rrmvergil~g lens with a divereinp lens having s diffweut dispersim. For e x a m p l ~ Figwc ~ 21H shuns m i arhi-on~nlof the type lieveloped by Frncntlmfer, in xhirh :L hirnnvrr ir,tivergiug lens of rroan glass is united with a plana-ronmve diverging i n s of flint glass. Tlre ronvergitig lrtis tends 11, forus lhe violet r a p in inmi ilf the red rays; the rliverging lens applies :t negative rotwctim tr, all these mys, with the encet heitig greater for the sho~.lcr wavelengths (,ti: Fip. 18). Bemuse t h e dispersion of flint glass is nmch grenler than that of r m w n ~ I B S S ,it is possible 1 0 elmoso IWRI length fur the diverging lens thni m n e s the violet and rcd i m n g ~ s to miwide, ahile only partially mnrdling the ranverpewe prodored by the fiW lens.
"i!
5000
C. A P O C H R O M A T
4000
FOCUS
+
Figure 21. Correction of chromatic aberration. Simple, uncorrected lens. Violet rays are brought to a focus closer to the lens than red rays. B. Doublet ienr containing two different glasrer of mequai dirperrions. Two different wovelengthr can b e brought to equol focus. C. Triplet lens of three different dirpenive media. Three wavelengths can b e brought to equal focus.
A
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No. 178 m Rmdar' Service Card ---t
Chemical Instrumentation \VIIPIIthe ioral Ienglhs of llle iwu lenses are SO c110sen as tcj make the red and violet, rays e s s d l y winvide in the image plane, other wavelengtt~s :%re not hn,upht to this same focus. That is. a two-piwe, or rlonblrl lens van rorrect only for two v r h s . The iwo rolors for whirh the lens is rorrerled ran he rhoson a t rill, and if they arc p ~ u p ~ r lspared y i l l the visible range, the nmount of residual rhromntic xl,erral i w observnhle visunlly is small. The rorrectirm ran he further improved h p t h e use of three lenses having differell1 dispersions, as iraliaated iu Fipme 21C. A triple1 can be color-corrected for three wavelenglhs: such lenses were first dpv i ~ e dby Ahhe and are d e d aporhromals (I.he prefix "apo" here means "free frumn-hence the lenses me free from color efferts; this is mpnnt to calry a stronger implication of success than that of the prefix "a," meaning 'without" in the term "arhromat"). The suhstanee fluorile is often employed as one of the
Figure 22. Multi-element magnifier% corrected for spherical and chromatic oberrotion. A. Model 3D3 binocvlor laupe of Edroy Products Co., New York. 17, N. Y. B. Spectacle magnifiers of Corl Zeirs, Oberkochen. C. Stereomosnifier of Tmpel. Inc., Fairport, New York.
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Chemical Instrumentation components of apochromatic lens systems. I n the process of achieving color correction, bot,h achromats and apoehramats correct a t the same time for spherical aherrstion, b y the principle outlined previously and demonstrated in Figure 12. The effect of chromatic and spheriosl correction can also be achieved if the converging and diverging lenses are not, remented together, as in the examples given above, but are separated from each other (though the required focal lengths would he different in the two cases). Thus, the divergingelement may be part of a separate eyepiece or ocular, which is used in conjunction with an objective lens eontsining the converging element(s).
Highly Corrected Mogniflerr When doing fine work at close quarters for extended periods of time, a moderate degree of abject enlargement that is completely free of distortion is often essential for precision and comfort. Examples of a type of binocular magnifier employing spherically and ehromaticdly corrected lenses are shown in Figure 12. The linear magnification is only 2 X to 4X. I n the case of the Zeiss spectacle magnifiers (Fig. 22B) the field of view is 35 mm s t a distance of 200 mm, and distortion is aoceptable for continuous work. The Tropel unit (Fig. 22C, %149) gives 4 X with 8, field of view of 30" and a working distance of 5.25 in.; the lens cells can he rotated upward over the forehead when normal vision is required. Zeiss also makes telescopic spectacles that e m be used as binocular magnifiers. The design af these s p e c t d e s is shown in Figure 23. The basic lens system eonsists of a two-element converging lens (8, in the figure) combined with s twoelement diverging lens (4 and 5). This gives a linear magnification of 1.8X withe. field of view of 23". For close-range work, an auxiliary lens (1) is slipped over one of the magnifiers (binocular vision is not possible now, because of the short distance necessary between object and lens), yielding magnificstions, depending upon the lens selected, ranging from 2 X to 6 X s t workine - distances of 223 mm to 72 mm. respectively. Magnifiers used for inspection and dimensional gaging of small parts must give images that faithfully reproduce the lengths, angles, and curves of the object. An inexpensive and imperfect approach that is frequently used is based on the Coddington lens design, illust,mted in Figure 24.4. The two spherical surfaces of the lens enclose s. diaphragm cut into the center of the unit; this stop limits the aperture to approximately the size of the eye pupil, and in this way the aberrations are kept within reasonable hounds. An illuminated magnifier based on the Coddington lens is shown in Figure 24B; in this case, the center cut serves as a reflecting and diffusing surfare for t,he (Continued on page A688)
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B Figure 23. Zeirr telescopic rpectocler. A. Actual appeorcnce. B. Optical schematic; 1 , dip-on reading lens; 2, objective ienrer; 3, intermediate ring; 4, 5, ocular lenses; 6, clamping ring; 7, rpocer ring; 8, $pectocle frame; 9, holding discs; 10, clomping ring.
light source as well as a. riiaphmgm to litnil, the aperture of the system. This is the desigu of the Bausrh and Lomh Illuminated Coddington Xagnifier ( 5 . 3 ) ; its n~ngnificlttionis IOX, mid its light sourre is powered by flashlight batteries. A more highly correrted magnifier illwrpwated in a flashlight illuminator is made by the Titan Tool Suppl>- Cn., Buffalo 16, N. Y., and is showti in Figure 2.5.1. The lens is a three-element a r h w mat, a r d the magnification is 1OX. 1-arious retir:les, shown in Figure 25.5, may be attached at the ahject plane of the lens fur measurement of the dirnensions of the object. The price of this device is 817 complete with m e retirle; the retirles are 93 earh. A similar instrument is the Bausch and Lomb IllurnC nated Hastings Triple Aplanntie llagnifier (SX, $25). A 7 X triplet lens mounted in a plastic reticle holder with a selertion of reticles, shown in Figure 26, is also svailable from this ma~mfacturer (designated the Measuring llagnifier, $14; with a set of four reticles, $36). A modification of the above illuminated magnifiers that is available from Bausch and Lomh is specifically designed for the (Contimed on page A834)
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Chemical Instrumentation
B Figure 24. The Coddington lens. A. The basic element has two spherical surfacer with a dioohmam cut into the aiorr between there surfacer to 1im:t the ooerrmionr. 8. The Bovrrn and Lomo I luminoled Coddngton Mognifer. in whim the oiophrogm i ~ r f o c nore rtdircd lo i vminale the object.
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comparison of two surfaces (the Surfare Con~parat,ar,S O ) . The optical stheme is shown in Figure 27; light from a. flashlight-type source is split into two h a m s hy s 45", partially-silvered mirror, and the reflected beams from both paths nre viewed simultaneously through the triplet lens at a magnification of 10X. An illuminated magnifier thnl provides a wide enough field of view to permit the object to be seen with both eyes, and henre with stereoscopic effert,, is the >lacroScope made by Ednalite Research Gorp.,' Peekskill, N. Y., and shown in Figure 28 (821:3 for magnifier, $75 for illuminated work plstfurm shown). If t,he top achromat lens is used alone, the magnifiration is 2.5X, and the field of view is a .5-in. dimmter cirrle. A second achromat may be inserted in a bayouet mount beneath the first lens to irmonse the magnification to 4X at the same viewing aperture. For st,ill higher mngnificzt,ions, a third lens may be swung into position beneath these two. The ohjert ran now he viewed through one eye, i.e., it is monocular, since the last lens has a smell aperture. If the latter is used in rombinntion with only the first lens, the mngnificiltion is 5 X ; if used in conjunr(Continuedon page A6.96)
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Figure 25. A. The T i t m Tool Supply to. Flarhlight Comporotor. 8. Reticles for incorporolion in the viewing rystem.
tion with both lenses, the total magific* tion is 8X. The specimen can be viewed by transmitted light, or by reflected light from a circular fluorescent lamp in the lens housing. Accessories are available
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for photomacmgvaphg, polarized light,
field, and color contrast work. Chemical h t ~ ~ m e n f a t i o n dark A sirnllar large field magnifier employ-
.~ .-
ing au nplnnatir and achromatic lens with an sportme of fiUx4", permit,tinp
tinn 2 x , mounted on a universal stand with fluorescent lamos in the lens hous-
For Lhe inspection and measurement of cavities, drilled holes, and recessed areas, a n instrumeut is xvailnhle from fision Engineering (the Bore hlagndier, 3258) Figure 26. The Bawch and Lomb Measuring Mognifler with core, with one reticle in the holder, ond three odditionol retider.
Figure 28. Cut-owoy view of the Ednolite MocroScope with its illuminated work platform. Figure 27. Optical design of the Bourch ond Lomb Surface Comparator for s i r n v ~ t . ~ . ~ ~ ~ that e n h d ~ e sa matic lens helow viewing of light reflected from two specimen*.
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l1I4' diameter achroahrch is mounted a
Chemical Instrumentation
/ Figure 29. Bore MogniRer of Vision Engineering inc. The lamp in the housing a t the rear is cooled by on air blower powered by a motor, both of which are contained in the housing. The stand bare contoins the transformer for the lamp.
semisilvered mirror. Light from a small, high intensity lamp is reflerted down d o n g the axis of the lens, and reflerted hark by the abject. Interchangenhle lenses are available with msgnifications from 1X t o 5 X ; the corresponding depths of field range from 3" to 6/s". The lamp is transformer-cont,rolled, and the intensity of illumination is adjustable ss required. The instrumen1 ia shown in Figure 2!l.
Figure 30. Model 239 Ramrden Hand Lens of Pociflc Tronrducer Corp.
A hand lens containing a highly rorrected bptieal system designed for 6 X viewing of objecls wit,hout distorlinn of dimensions or angles is the >lode1 230 ILamsden Hand Lens ($4) of Pa
