LeRoy A. McGrew ~ d l State l University Muncie, Indiana 47306
Stereographic Slides of Molecular Models
Stereographic slides of molecular models can be used to create three-dimensional images of molecular models and are thus useful visual aids for the study of stereochemistry and couformational analysis. Such slides may be presented to students individually with the aid of stereo viewers, or they may be presented to ? mass audience in the classroom with the aid of suitable projection equipment. Since the magnificatioq of the image does not depend on the size of the original model, but rathe; on the capabilities of the v:ewing or projection equipment, the use of such slides for the study of molecular structure offers certain advantages over the use of actual models. AIolecular models large enough for lecture demonstration may present problems of storage and assembly. The inexpeqsive student model kits often lack versatility, or they may require considerable time for constrnction and disassembly of complex structures. In the teaching of many strncturalconcepts these difficulties may he avoided, without loss. of illustrative value, by substituting a photographically-produced stereo image for the actual model. The stereo daguerreotypes still in existence attest to the fact that stereo photography, or "stereoscopy," is as old as photography itself. The application of stereoscopy to educational situations also represents nothing new. However, the considerable interest in this technique shown at one time by amateurs has declined to a very low present level. Stereo photographic equipment has not developed to the same extent as non-stereo (planar) equipment, and, except for slide mounts and viewers, has largely disappeared from the market. I would like to describe here a procedure which an instructor may follow in order to prepare stereo slides applicable to chemical education. The procedure allows the use of modern planar photographic equipment and results in the production of quality three-dimensional images in full color. This report is particularly concerned with the preparation of slides for individual student viewing, rather than for classroom projection. Photographic Equipment
A stereo slide of any molecular model can be made by taking two pictures of the model from two different camera positions, much in the fashion that the human eyes would see two slightly different views of the same object. The resulting transparencies constitute a "stereo pair" in which there is a parallax of correPresented at the 160th Meeting of the American Chemical Society, Chicago, Ill., September, 1970.
sponding points due to the lens separation of the two camera positions. When t,he two frames of the pair are mounted in a stereo slide mount and viewed in a stereo viewer each eye sees only the image meant for it, the two images are fused, and the three-dimensional image of the model is re-created in the mind of the observer. The stereo cameras which were once readily available were equipped with two lenses and took both frames of the pair simultaneously. This was a necessary feature for the accurate portrayal of scenes in which motion was present. It is not generally known, even among camera store personnel, that a two-lens stereo camera is not necessary for the preparation of the type of stereo slide under discussion in this report. Since a molecular model may be held stationary during the taking process, the two frames of the stereo pair need not be taken simultaneously, but may be taken in succession with a planar 35 mm slide camera. We may even say a t this point that a two-lens stereo camera is generally unsatisfactory for this kind of photography. Such cameras have a fixed separation between the lenses of 69 mm, which is about the same as the average human interocular separation. This separation is appropriate for normal outdoor photography, but is generally too wide for the close-up photography of models. The use of a separation of this magnitude when photographing models from a distance of about 4 ft or less will result in distortion of the depth characteristics of the model. Infusability of the pair and eyestrain may also be experienced by the observer. A planar camera, for which the lens separation may be varied and accurately controlled during the taking process, is thus preferable to a stereo camera for this kind of photography. An added advantage of the planar camera is that both frames of the pair are taken with the same lens, insuring that the only difference in the two frames will be the necessary parallax introduced by the separation of the two camera positions. To insure accurate centering of the model in the field of view a single-lens-reflex 35 mm camera is desirable. The normal lens of the camera will work fine, so long as it is capable of focusing to the relatively short distances required to fill the frame with conventional molecular models. The author bas used a Minolta SRT-101 35 mm camera equipped with the standard f/1.7 lens of 55 mm focal length with excellent results. Exposure setting is a simple matter since this camera is similar to most of the newer single-lensreflexes in having a CdS exposure meter situated behind the lens. The camera with cable shutter release is mounted on a tripod before the set, which is provided with a Volume 48, Number 8, August 1971
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pastel poster board background. Auxiliary lighting is provided by two No. 1 photoflood lamps in 12-in. reflectors. One lamp is placed in the plane of the model and perpendicular to the camera-model axis. The other illuminates the model frontally a t an angle of 45" to the camera-model axis. Although this light setup does not completely eliminate background shadows, the shadows are not distracting in the final stereo slides. Kodachrome I1 Type A Professional film (ASA 40) is balanced for use with the No. 1 photofloods and gives good color rendition in this situation. The film is available in 36-exposure rolls from which 18 stereo pairs result. In stereo photography it is desirable to insure sharp focus of all details in the subject, and this is accomplished by using a relatively small aperture setting in combination with a relatively slow shutter speed. Apertures in the range f/S-f/ll coupled with a shutter speed of sec constitute normal exposure conditions with the lighting and film described above and provide good depth-offield in the final transparencies. So far only conventional close-up photographic technique bas been described. For stereo representation some means of accurately changing the camera position between the two exposures of the stereo pair must be provided. A rectangular bracket (See Fig. 1) with a 1/4 in. slot milled in the top may be constructed and used for this purpose. The bracket is bolted to the tripod, and the camera in turn is mounted on the bracket by means of a 1/4 X 20 bolt through the slot into the standard tripod socket on the base of the camera. A centimeter rule with a zero point in the center is scribed along the back of the slot and indexes with a mark on the back of the camera body. Inter-lens separation is thus adjustable over a range of values and can be accurately controlled and reproduced. The dimensions of the bracket are not critical, but the slot should be long enough to allow lens separations of a t least 4 em. A rack-and-pinion device such as that shown in Figure 2l is an alternative to the bracket, but is considerably more expensive. Since the camera is mounted on the platform of the rack-and-pinion unit, linear horizontal movement of the camera is insured. With either method, the worker must level the mounting device prior to the taking
Figure 1. Bra
