Three-Dimensional Pointers for Stereoscopic Projection H. J. G. Hayman The Hebrew University, Jerusalem 91904, Israel
Stereoscopic projection involving separate pictures for each eye is often used for illustrating molecular structure hut suffers from the disadvantage that an ordinary pointer cannot he used for indicating a particular atom since each observer would see such a pointer in a different position with respect to the stereoscopic This difficulty can be overcome by using stereoscopic prijection not only for the molecule but also for the pointer itself. We describe here three different setups we have used for this purpose. The most generally useful of these is shown schematically in Figure 1.Light from two matched V-filament lamps V (3-V Rowi arrow bulbs, no. 528) is polarized by the polaroid filters P and focused on the screen by two achromatic lenses L (36-mm diameter, 100-mm focal length). The lamp carrier C is moved backwards and forwards hv means of the focusing knoh F until sharp images of the f filaments are obtained on the screen. If necessani. the l a m carrier C is tilted by means of the screw S until these images are a t the same height. Using appropriate polarizing glasses, an observer sees a single V filament suspended in space a t a distance from the screen depending on the distance between the two images on the screen. This distance can be varied by changing the distance between the two lenses L bv means of the lever and wedge mechanism shown in the diagram; this is accomplished by rotating the knoh K. Since'the knob K sewes also for rotating the apparatus about vertical and horizontal axes passing through its center of gravity, the lecturer can readily make the V-filament image coincide in space with any desired part of the simultaneously projected stereoscopic image of the molecule. Ghost images of the V filaments sometimes appear as a result of slight depolarization of light by the screen; they can he rendered innocuous bv reducing the lizht intensitv of the V-filament lamps by means of a rheostat. The three-dimensional oointer described ahove has also been used very successfuliy with anaglyphic projection by replacing the two polaroid filters, P , by red and hlue-green filters, respectively. The second of our three-dimensional pointers was designed specifically for use with our douhle-headed overhead projector2,$ the principle of this pointer is shown schematically in Figure 2. The two stiff wire pointers P, one for each eye, are projected along with the corresponding pictures of the stereoscopic pair so that the observer sees a single pointer in space pointing to some part of the projected stereoscopic imaee of the molecule. The ooerator can use this oointer to u indicate any desired position in space by moving the pointer carrier backwards or forwards on the slides S and sidewavs on the rods R and by adjusting the distance between the twowire nointers bv means of the lever L. he set"p actually used differed somewhat from that shown in Figure 2. The oarallel rods R are mounted verticallv one above the ot,her so that the axis of the toothed wheei T is horizontal; this wheel is rotated by means of a knob rather Presented at the 50th Anniversary Meeting of the Israel Chemical Society. Jerusalem, April 1984. McGrew, L. A., J. CHEM. EDUC., 49, 195 (1972). Hayman, H. J. G., J. CHEM. Eouc., 54, 31 (1977). Somewhat similar projectors are available from R. Schmidt. Treppenstr. 9.D-6109 Mllhltal 1, W. Germany.
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L Figure 1. Thraedlmensional pointer based on two V-filament lamps.
Figure 2. Threedlmenslanal pointer for use with a pmjector.
double-headed overhead
than by the lever L shown in Figure 2. The two slides S consist of a oair of sliding drawer carriers mounted on a rieid metal frake which fits i n t o the top of the overhead proje&or. This frame carries a horizontal elass olate a few millimeters above the glass stage of the overKead projector. The transparencies are placed on this glass plate while the pointers P (2-mm diameter with pointed ends) move between this plate and the overhead ~roiectorstage. This three-dimensional pointer is extremel;effkctive since its position can be seen a t a glance; unfortunately it can only be used with a double-headed overhead Figure 3 illustrates our third three-dimensional pointer which was designed for pinpointing individual atoms when using stereoscopic projection for illustrating the structure of complex molecules, such as proteins. Light from a He-Ne laser (Spectra-Physics model 155,0.5 mW) is reflected vertically upwards by means of a totally reflecting prism P1 and then reflected horizontally hy a second prism P2. This horizontal light heam is then split into two approximately parallel beams by means of a heam-splitter B and a third prism P3. The beam Volume 61 Number 12 December 1984
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the second beam, reflected by the prism P3, reaches the screen via the concave lens L3 (-1 diopter, 42-mm diameter) and the polaroid filter F3. We now have two red light spots on the screen; these are brought to the same height by raising or lowering the lens L1 by means of a screw (not shown). When viewed through polarizing glasses these two spots coalesce to give a single spot a t a distance in front of (or behind) the screen depending on the distance between the two spots on the screen. This distance can he varied smoothly and continuously by rotating the control arm A so as to give a lateral movement to the lens carrier C carrying the lenses L2 and L3. The control arm A serves also to rotate the apparatus about the two axes of rotation coinciding with the vertical and horizontal parts of the laser beam (V and H, respectively) so as to hrine the lieht soot to anv desired Dart of the stereosconic image of the molecule. The rotatable disk D carries two oolaroid filters F1 (the polarization axes correspond to the direction of the shading in Fie. 3) and two oDen windows W. The laser beams normallv psssthrough thesewindos,~,hut ifghost imagesareobsen~ed, they can be largely eliminated by rotating the dick D so that the laser beams pass thrvugh the polaroid filters I;l, thus reducing their intensitv. This rotation is effected bv means of the r d R, connected to the disk via a fork so as allow the lieht beam to reach the prism P3. Finally, a word of warning: whereas in scereoscopic projection it isalwavs desirable that the two images oieach atom should appear on the screen at exactly the same height, this is especially important when using a three-dimensional pointer, particularly if this is of the V-filament or laser type.
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m
He-Ne LASER
m
Figure 3. Threedimensional pointer using a laser beam. Above: Horizontal cross-section of optical system. Below: Light regulating disk and lens carrier as seen after removing back cover.
reflected by the beam-splitter passes through the concave lens L1 (-2 diopters), the convex lens L2 (1 diopter, 42-mm diameter), and the polaroid filter F2 hefore reaching the screen;
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Journal of Chemical Education
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