The Mounting of Stereo Slides for Projecting Molecular Models H. J. G. Hayman The Hebrew University, Jerusalem 91904, Israel
I have noticed that the mountinaorocednre used bychemists when showinga sequenceof 3- slides sot ten leaves murh to be desiwd; either the proiectors have to he readjusted at frequent intervals, or the-audience is forced t o view many of the slides under adverse conditions. Stereoscopists have realized for some time that such procedures cause visual discomfort and that this can only be avoided hy careful mounting of the 3-D slides; it is then necessary only t o adjust the projectors for the first stereo pair of the sequence, after which the remaining oairs will automaticallv he oroiected satisfactorily. Such iaieful mounting is insisted uponin the exhibitions oreanized hv the various stereoscopic societies and forms an important criterion when judging stereoscopic competitions. As a consequence of this, the stereoscopic litera&re is full of instruckons for the correct mounting of stereopairs. However, for achemist wishing to project stereo slides of molecular models, much of this material is irrelevant and misleading, and this is for two reasons. Firstly, the stereoscopist is primarily concerned with the correct position of the "stereo window" through which the 3-D scene is viewed. whereas the chemist either sees no window (for diagrams having a black background) or ignores i t since its position is, for him, of secondary importance. Secondly, many of the usual technical methods for positioning the film chios are unsuitable and often useless when dealina with steieo pairs illustrating molecular structure, especially if these have a hlack background. My experience is that when projecting 3-D pictures of molecules, eyestrain will he minimized if the molecule is positioned just in front of the screen with the farthest atom in the plane of the screen. This arrangement greatly simplifies the correct oositionine of the film chins in their mounts. We describe heie the mo&ting of the left'and right views in seoarate 50 X 50-mm slides hut the method anplies .. equallv . . well if the two views are to he mounted in a single slide. The fieure shows the correct mountinr! of a stereo pair. The distance x from the frame to the fakhest atom of the molecule should be the same in both slides; this ensures that if the projectors are adjusted so that the farthest atom in the first stereo pair of a sequence lies in the plane of the screen, Furthermore, the then this wii1 hold for ail subsequent heights of corresponding points in the two slides should, as far as possible, be the same; one method of achieving this is to see that the heights y, and yz of two suitably chosen atoms are the same in each slide. These atoms should clearly be chosen reasonably far apart, horizontally; furthermore, it is good practice to choose atoms that are vertically near the center of the molecule. (This precaution is advisable if the disparity between the two pictures can be ascribed to a rotation of the molecule hut is unnecessary if the disparity corresoonds to a linear disolacement of the observer.) ~ ~ ' avalues l of yl and y? in the two slides can be checked by direct measurement, but the following alternative method of obtaining equal heights of corresponding points in the two slides is simpler and more satisfactory. In this method, the comnuter used for drawine the stereo nair . .d o t s two supplementary points, A and B, a t the same height in approoriate nositions in each of the two oictures. The film chips are then positioned in the corresponding slides such that the
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stereo pair of hexahelicene illustrating me correct positioning of the fllm Chips with respect to the picture frames. A
points A and Bare just visible above the lower boundaries of the picture frames. A similar method can he employed when the slides are prepared by photographing an existing stereo diagram; in this case, the points A and U should he marked cnrefully on the stereu diagram, the relatively large scale of which enables this to he done wirh sufficient accuracy. Althoueh - theadiustment coohtain eaual values uf x in the two slides can be made by direct measurement, a more satisfactow method is to insert the film chips into the lower halvesof the two slides and then t o clampthe latter, side by side, on alight boxso that they can heviewed stereoscodcally. A simple stereoscope withlegs removed, mounted tb give freeaccess to theslides, is very suitable for this purpose. The chips are adjusted laterally until the farthest atomis seen to lie in the plane of the picture frame and are then fixed in nosition bv small oieces of adhesive tane. The slides are then completei by adding their upper hal4es. I t should be noted that this method can even be used for stereo oairs consistine of light lines on a hlack background since suck a hackgrouni is not comoletelv . ooaaue: . . however. for such stereo pairs the ambient light level must be kept very low, otherwise the film boundaries will be visible and cause confusion hv aooearina as an apparent picture frame. "Gepe" glass sl~de-&ounts~ recommended in the stereoscooic literature, are very suitahle for the atxwr prwedures sinre the retaining tabs help prevent inadvertent movement of the film chip.; while their positions are being adjusted. With the mounting and projecting techniques described above. the nroiections of the oicture frames of stereo nairs consisting of dark lines on a light background will coincide on the screen, that is, the stereo window will lie in the plane of the screen. This eliminates ghosting round the picture frame that otherwise may appear as a result of imperfections in the polarization system. Furthermore, under these conditions, the "shadow pointer" described by Tollenaere' can he used satisfactorily provided the left-hand projector is used to project the lefthand slide. (This pointer cannot be used if the projectors are mounted one above the other; for use with stereoscopic pictures positioned behind the screen, the lefthand projector must he used for projecting the right-hand slide.) Volume 64
Number 12 December 1987
1041
Stereo pairs of molecular structure printed in the literature are usually drawn for a n angular disparity of around,'6 corresponding to a linear displacement of about 50of the distance of the viewpoint from the center of the molecule. Slides made from such drawings are reasonably satisfactory for small audiences near the projection screen hut give exaggerated depth a t greater distances from the screen. For this reason, as mentioned p r e v i o ~ s l yi,t~is preferable for larger audiences to use an angular disparity between the stereo pairs of only 2 O (corresponding to a linear displacement of approximately 1/30 of the distance of the viewpoint from the molecule-the well-known "one-in-thirty" rule of the stereoscopist)?
1042
Journal of Chemical Education
One further point, the International Stereoscopic Union's Standard for distinguishing between the left- and righthand slides of a stereo pair is to mark the bottom left-hand corner of the slides (when positioned for direct viewing) with red and green spots, respectively. (If the stereo pair is mounted as a single slide, a single red spot suffices.) However, since the opposite convention has also been used, I mark the bottom left-hand corners of my slides with a red L or a green R in order to avoid misunderstanding. Tollenaere, J. P. J. Chem. Educ. 1985, 62, 701. Hayman, H. J. G. J. Chem. Educ. 1977,54,31. Ferwerda,J. G. The Worldof 3L;Netherlands Society for Stereophotography: Borger, Holland. 1982: p 104.
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