Henry Hubinger and Harry P. Schultz University of Miami
Coral Gables, Florida 33124
Time-lapse Multiple Slide Projection
The slide projector has long been a wellknown and extensively used instructional aid. The purpose of this paper is to present the details of a sophisticated modification of the simple slide projector, and to discuss its use in the realm of chemical education. The device, time-lapse projectors, projects upon a screen the image from a slide. Activation of a slide change button causes the first image to dim and disappear while simultaneously the image from a second slide is projected in the same area that was occupied by the first image. The second picture brightens as the first one dims. The image-replacement cycle takes 4 sec, and is then stable and constant until the change button is again activated. Time lapse projection is effected with two 2 X 2-in. automatic slide projectors mounted side-by-side or vertically above one another, aimed and focused on the same area of the screen. For this purpose the carousel types of projectors equipped with zoom lenses have been found most useful; they may be mounted close together with consequent minimal parallax effect. The projectors are then connected, via an electronic control unit, to the "Kodak Dissolve Control for Carousel" (Price, $150.00). The advance button is connected to the electronic control unit. Depression of the advance button dims and turns off the first slide projector as a slide is advanced in the second slide projector, with brightening of the projection hnlh
The time-lapse projection method possesses the advantage of using readily available 35 mm projectors. A small wooden frame to support the two projectors, an electronic synchronization unit, a stable projection platform, and a screen are additionally required (Fig. 1). In order to utilize the time-lapse technique of slide projection, a modicum of photographic equipment is also necessary for the preparation of slides. A single lens reflex (SLR) 35 mm camera with a universal focus adapter for the lens is a szne qua non for the preparation of slides. A sturdy mount consisting of a ring stand bolted over a 2 X 2-ft platform (or its commercial equivalent), with an attached universaljoint camera holder, is equally necessary. Cable release, tripod, and lights are useful additions. In clement weather the photographic work can be executed with sunlight, preferably on the north side of a Presented at the 23rd Meeting-in-Miniature of the Florida Section, American Chemical Society, Cypress Gardens, May 8, 1970. 'HOBHALL, E. M., J. CHEM.EDUC.,11, 21, 23, 154, 235, 546 (1934).
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Figure 1.
I l s t r ~ d t i ~ l tAid ll
of projectorr for time-lopre
multiple slide projection.
building. Color slides developed and mounted for use cost about 20$ each. Preparative time is extensive for this work; a week of a teacher's "spare time" may be easily utilized to prepare the titles, drawings, and models needed for a 20-frame sequence. Various aids for some phases of this work have been described.' Time-lapse projection apparatus has been found to be especially useful to convey certain dynamic and/or spatial concepts of chemistry and physics to beginning classes of inorganic and organic chemistry. Brief presentations of three applications of timelapse projection illustrate the utility and mode of application of this educational technique. Students readily grasp the concept of permanent dipole-dipole attraction; it is a relatively static concept and is readily described u~iththe aid of blackboard, overhead projector, or traditional slide projector. However, London Forces, the attractive forces between nonpolar atoms or molecules which are the consequence of transient polarizabilities and induced dipoles in adjacent units, present a more elusive concept to students. A series of 22 slides has been used to illustrate this dynamic concept. Few words are needed; the slides are virtually self-explanatory, and may be rapidly projected one after the other. Students grasp the concept in about three minutes. Two title slides with the key words "London Forces" and "Transient P~larizabilit~" precede a third slide that illustrates the increase in attractive forces between nonpolar atoms or molecules as a consequence of increasing electron content of the structural units. The third slide projects the image of a line graph showing boiling point versus electron content for the inert gases (inorganic chemistry classes), or for a few simple alkanes (organic chemistry classes). A fourth slide projects the pattern which defines a nonpolarized and polarized unit (Fig. 2); brief comments clarify the significance of the drawings. In rapid sequence a time-
Figure 2. Introductory slide in rerier on London forces which defines a polarized ond nonpolarired unit.
Figure 4.
Figure 3. Subsequent slides in the series on London forces showing progressive transient polorimtion.
lapse series of slides depicts an aggregate of 5 nonpolar units (Fig. 3) as progressive transient polarizability appears and induces dipoles in adjacent units until all units are polarized. A return to nonpolarized status is projected, followed by a similar series of five different slides, with opposite charges at each end of the transient dipoles. A third five-slide series initiates the dipole in the center of the five-unit drawing. Care must be taken to make the drawings of the same size, centered in the same positions on each of the identical cards or sheets used for the drawings. Moreover, the pictures should be taken under identical circumstances, maintaining camera and drawing in the same relative positions to one another through the entire series. Lack of positional reproducibility causes the series to move up and down and/or from side-to-side on the screen as one slide replaces another. This is disconcerting and detracts from the clarity of the concept. A second series of slides illustrates the spatial, or three-dimensional, concept possible of emphasis by this mode of projection. Beginning students invariably
Series of slider showing the equivalency of the octahedral axes.
encounter difficulty in understanding the concept of the octahedral structure. Oddly, the two horizontal axes of the octahedron are readily appreciated to be identical, but the vertical axis (Fig. 4) appears to most students to be of a different relationship to the entire figure. A construct of the three perpendicular axes of an octahedron, each axis of a different color [red (R), white (W), and blue (B)], is aligned on a support before the camera. Color transparencies are taken as each axis is, in turn, displaced from the vertical to the horizontal, tilting the model 15-20' with each new photograph. These slides, projected in time-lapse sequence one after the other, convince the viewer that all three octehedral axes are equivalent. The third and last example of the use of time-lapse projection technique simultaneously illustrates both dynamic and dimensional concepts. All organic chemical rearrangements possess both dynamic and dimensional qualities. The pinacol-pinacolone rearrangement is especially well illustrated by the multiple projection, time-lapse technique of slide projection. This series of slides commences with a title, a specific example, and the various mechanistic steps involved in the rearrangement. Utilizing a threedimensional projection drawing (Fig. 5), the same mechanistic sequence is projected and commented upon to prepare the students for the sequence of twenty slides which portray the rearrangement itself. The rearrangement sequence was prepared by photographing ball-and-stick models wherein each different kind of atom was identified by a different color. Extraneous hydrogen atoms were omitted from the models. The color slides show the pinacol molecule
Figure 5. Slide showing o three-dimenrionol projection drawing of the mechonirtic sequence of the pinocol-pinocdene rearrangement. This serves ar an introduction to the sequence of twenty slider which show the rearrangement using ball-ond-stick models.
Volume 48, Number 9, September 1971
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B
~i~~~~ 6. A portion rearrangement.
D
the slide sequence showing the pinocal-pinacolone
assuming the necessary staggered, trans-coplanar relationship between leaving and migrating groups, protonation of an hydroxyl group, and its subsequent loss as water, with assumption of a planar character by the consequently generated carbocation onto which the adjacent group migrates. Figure 6 pictures a portion of this sequence. At this point the model is rebuilt with a specially bored trigonal, planar carbon atom inserted into its framework. The sequence is completed with appropriate replacement of the planar carbon atom by an inverted tetradedral carbon atom, and final use of the triaonal hybridized carbon atom'as the carbonyl carbon in the rearranged pinacolone product. Removal and migration of the various groups is pictured in three to four frames for each movement, progressively lengthening the appropriate peg prior to removal-of a group.
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A smooth presentation requires careful and repeated positioning of the skeletal carbon atoms as each picture is taken. This was done by fixing the oamera above a white cardboard background which had attached to its rear edge a second perpendicular cardboard background. Penciled coordinates on each cardboard enahled ready~. replication of the nositions of the skeletal carbon atoms throughout t6e series. At some stages of the sequence it was necessary to use tiny blobs of silicone putty and/or bits of tooth picks to prop the carbon atoms in a stable, reproducible position for ~ h o t-o ~.r a"~ h v . ~ x ~ e r i m edemonstrated tk that compact scale models of molecules had their structural units so close together that confusion resulted in the mind of the viewer when these types of models were used as models for the photographs. The ball-and-stick models lent themselves both to read? Comprehension by the viewer, as well as to the minor structural modification necessary on some portions of the molecule for detailed display of each facet of the rearrangement. In conclusion, not all presentations need nor should be handled by time-lapse projection. Static displays, two-dimensional projections, lists of data-all these are more simply handled by use of a blackboard, overhead projector, or ordinary slide projector. However, dynamic and certain three-dimensional concepts are more clearly and more rapidly elaborated by use of time-lapse projection. Acknowledgment
The authors acknowledge with gratitude the help and constructive suggestions from the Visual Aids unit of the University of Miami. In particular the art and arrangement work of Richard Coram and Carol Levy are deeply appreciated.
