THIS MONTH I shall discuss some classic visual toys that appeared in the l9th century. Although a few of the toys are still sold, most of them have fallen victim to newer entertainments. They are so diverting and instructive that they deserve a better fate.

One of the simplest of the visual toys involved what are now called moiré patterns, the shimmering effect one sees when certain regular patterns are laid one over the other so that in some places they are in step and in others they are out of step. A simple illustration of the effect can be seen with a haircomb. Hold the comb parallel to a mirror so that you can look through the teeth and see the image of the comb in the mirror. If the comb is the right distance from the mirror, some of the teeth in the image will coincide with the real teeth in your field of view. Other teeth will be exactly out of step with the real teeth. What you see is a periodic pattern along the comb's length. The pattern is darker where the teeth are out of step. If you move the comb parallel to the mirror, the periodic pattern of darkness (assuming the comb is dark) runs along the length of the comb in a wavelike motion.

This kind of motion was employed in illustrations published in the 19th century. A black-and-white illustration was drawn with many closely spaced parallel lines. The reader was told to place over the drawing a screen that was also covered with evenly spaced parallel lines. I am not sure what the screen was made of; it could have been glass or some kind of translucent paper. When the viewer slowly moved the screen over the drawing, the moiré pattern that developed between the screen and the drawing moved across the drawing, giving the illusion that objects in the drawing were moving.

Dover Publications has reproduced (under the title The Magic Moving Picture Book) a book that was originally published in London in 1898 as The Motograph Moving Picture Book. The transparent screen is acetate ruled with closely spaced lines. The pictures include geometric designs, a representation of an erupting volcano and a scene in which a fireman directs water at a fire. When you move the acetate over the picture of the fireman, for example, the moving moiré pattern makes the fire shimmer, the smoke rise and water on the street flow along the pavement. In other illustrations the moiré effect seems to make wheels turn and water flow past a ship.

You can easily duplicate the effect for yourself. Draw on a sheet of paper a geometric design consisting of many evenly spaced parallel lines. Then rule similar lines with comparable spacing on a sheet of transparent acetate. The acetate can be bought in an art-supply store, where you can find out what kinds of ink adhere well to acetate.

After the ink dries place the acetate on your picture and move it slowly across the picture with the lines in the acetate and the picture parallel. The pattern in the picture will wiggle and squirm. You can try several variations of the trick. Make the lines on the paper slightly more or less widely spaced than the lines on the acetate or make them wavy rather than straight. Try orienting the lines on the paper at angles different from the ones on the acetate and making composite figures consisting of areas with different orientations of lines. Finally, you can draw scenes in which only the appropriate areas show motion.

A related toy was marketed recently through "7-Eleven," the national food-store chain, to promote a beverage. The toy, now under patent application by Magic, Magic, Magic, Unlimited, consists of two plastic cups that are nested and can be rotated individually. The outer cup is clear plastic with an area of vertical black stripes. The opaque inner cup bears two superposed drawings consisting of vertical stripes the same width as those on the outer cup.

With one orientation of the inner cup the black stripes block all the vertical segments of one of the drawings and expose all the segments of the other drawing. From a distance of a foot or so the viewer sees a nearly complete picture because the black stripes are apparently ignored and the viewer mentally completes the picture across the stripes. By rotating the inner cup the viewer can replace the exposed segments of one picture with those of the other. Slowly continuing the rotation alternates the exposure of the two pictures. On my cup a baseball player is either bending down to field a ball or is standing. When I rotate the inner cup, he appears to alternate between the two poses.

A large variety of changeable pictures were available in Victorian times. The simplest kind involved nothing more than a picture that had several slits through which segments of an alternate picture could be slipped. Motion was better suggested by the slipping slides developed for the "magic lantern," which today is called a slide projector. These consisted of a glass slide with a scene painted on it and a second slide that could be slipped in front of the first one. The second slide bore the element of the composite picture that was to be moved. When the slide was projected, the operator could slip the second slide over the first to give motion to the projection.

You can easily duplicate this system with most slide projectors. In the book Making Victorian Kinetic Toys Philip and Caroline Freeman Sayer suggest using the thin sheets of glass from the mounts for 35-millimeter slides. Buy a translucent paint that adheres to glass and does not degrade in the heat of a slide projector. Paint the fixed scene on one sheet of glass. You may want to trim the other sheet so that there is more room for sliding it. Attach to that sheet a convenient handle, such as a plastic toothpick, so that when both sheets have been inserted into the slide holder of the projector, you can move the second sheet by the handle.

One Victorian visual toy, the kaleidoscope, is still popular. It was invented (or at least patented) in 1816 by David Brewster, who is best remembered for his work in classical optics and particularly for the Brewster angle of the polarization of light by reflection. Although there are many variations on the kaleidoscope theme, the basic design is a tube containing two rectangular mirrors or pieces of shiny metal taped together at an angle of about 45 degrees with the taped edges running the length of the tube. At one end of the tube is a compartment containing bits of colored glass or other material. At the other end is a small hole through which the viewer peers. One can see a pie-shaped section of the bits of material at the other end and reflections of the same segment in the mirrors. When the tube is rotated, the symmetrical pattern shifts as the bits of material fall into different configurations.

Designs for a homemade kaleidoscope are available in Carson I. A. Ritchie's excellent book Making Scientific Toys. Cut off the bottom of a fairly long plastic bottle with a narrow neck Opening. The bottle will serve as the tube and the opening will be the aperture through which you will look. Inside the tube will be two rectangular mirrors with their edges at an angle of between 40 and 70 degrees. The bottle should be about a centimeter longer than the mirrors once they are fitted in.

Put the cutoff end of the bottle on a sheet of 1/8-inch Plexiglas, trace a circle on the Plexiglas around the circumference of the bottle and then with a jigsaw cut out the circle of Plexiglas. File the edge until the piece just fits inside the bottle.

Hinge the two mirrors together with strong reinforced tape. Put the Plexiglas piece on a table, stand the mirrors upright on it and fasten them to it by gluing some small wood blocks into the angle between them and the piece. The blocks will also fix the angle between the mirrors. Your results will be best if you have positioned the mirrors so that both edges of each touch the circumference of the circle. When the glue of the assembly has dried, push the assembly into the bottle (Plexiglas end last) as far as it will go. Now make another Plexiglas circle of the same size, but instead of leaving it transparent rub it with steel wool to make it translucent. This piece will be the end of the kaleidoscope.

To make a compartment for the bits of colored glass cut out a strip of stiff cardboard that is long enough to be fashioned into a ring to fit in the bottle between the two Plexiglas circles. The cardboard should not stick out of the end of the bottle. Glue the ring to the translucent piece of Plexiglas. Put into the ring a few bits of colored glass and slip the ring into the end of the bottle. You now have a kaleidoscope with a changeable display.

To investigate the effect of the angle between the mirrors you might prefer to work with a simpler arrangement. Hinge the two mirrors together and stand them upright on a geometric design drawn on a piece of paper. Look down into the trough formed by the mirrors to see the kaleidoscope design. How many pie-shaped sections do you see for a given angle between the mirrors? Try varying the angle from the smallest one you can achieve to one as close to 90 degrees as is possible, recording the number of images as a function of the angle. Do all the pie sections have the same angular width? In particular, is the pie section lying behind the hinge of the mirror the same size as the other sections?

One of the best kaleidoscopes I have found has a lens at the far end instead of a container. (I think the device is properly called a teleidoscope, but I am not certain.) The focal length of the lens is less than the length of the tube. When a fairly distant object is viewed with the device, the image size is reduced by the lens so that many such objects can be seen simultaneously. To me the effect is beautiful, since it is brighter and more colorful than the images in a standard kaleidoscope, where the bits of colored glass and the translucent backing reduce the intensity of the light. If I watch passing scenery with the device as I ride in a car, the constantly varying display is almost psychedelic.

You can make this kind of kaleidoscope out of a cheap lens, a cardboard tube and the usual two mirrors. Choose the lens and the tube so that the focal point is about halfway down the length of the tube. The opening at the viewing end of the tube should be small but should still be large enough for you to see all the image, including the reflections in the mirrors.

An enormously popular diversion in the late l9th century was viewing photographs in a stereoscope. In his book Ritchie gives plans for making replicas of the old stereoscopes. You can find old stereoscopes and stereo cards in antique shops, and you can buy replicas from

Stereo Classics Studio, Inc., 145 Algonquin Parkway, Whippany, N.J. 07981. The commonest kind of stereoscope has two lenses, one for each eye, through which a viewer sees two pictures, one for each eye. The pictures are not identical but give slightly different perspectives of the same scene to match the perspectives each eye would see in the same setting. Hence when the pictures are fused by the stereoscope, they are seen in the original three dimensions.

Budd Wentz describes a simple procedure for making a different type of stereoscope in his book Ready-to-Make Photo & Scene Machines. The viewer looks through a piece of cardboard resembling a Halloween mask. One purpose of the mask is to eliminate distractions so that the viewer can concentrate on details that help him in assigning depth to the objects in the picture.

In the center of the mask there is a length of stiff cardboard that has a flat mirror glued or taped to one side. The idea is to view one picture directly with one eye while viewing a reflection of the other picture in the mirror with the other eye. This system forces the eyes to converge, which in turn helps the viewer to assign depth to the pictures. As before, the two pictures show slightly different perspectives, but in addition one of them must be "flopped" (left and right reversed) so that it has the same orientation as the other one after reflection. If everything is done right, the viewer fuses the two images to get a single three-dimensional image. Wentz provides reproductions of some of the stereo cards that were popular with this type of device in the late l9th century and early 20th.

The standard stereoscope available in antique shops is different from Wentz's in that it has two wedge-shaped lenses to facilitate the merging of the different images seen by the viewer. Both pictures have the same orientation, since no reflection is involved. This kind of stereoscope was apparently developed in 1859 by Oliver Wendell Holmes, partly on the basis of earlier work by Brewster. Photographs were usually made with a camera that had two lenses, which were separated from each other by about the distance between the eyes of an average person. The focal lengths of the lenses in the camera and in the stereoscope were the same, so that the viewer saw the same perspective the camera did. The photographs could also be obtained with an ordinary camera by making one photograph and then moving the camera seven or eight centimeters to the side to make the other.

Thomas B. Greenslade, Jr., and Merritt W. Green III have published an account of their experiments with this type of stereoscope and their attempts at making photographs for it. You might like to follow up on some of their work either with an antique stereoscope or with a stereoscope you have made. If you make one, Ritchie suggests that you have an optician grind the lenses. His specifications for the lenses are "power approximately flat B 1/SPH + 5.50, lens cut on optical center, 8 base out E/E 47 pound blank." You might try to work with some wedge-shaped glass prisms instead.

Greenslade and Green were able to make their photographs with a Polaroid camera (Model 180) that had a lens with a focal length close to that of the antique stereoscope they had bought. In one experiment they wanted to ascertain how the lateral displacement of camera positions between successive photographs affected the quality of the three-dimensional illusion. Ideally the displacement should match the distance between the viewer's eyes, but depth appeared in the pictures even when the camera displacement was more than or less than the ideal distance. With a large displacement, however, the objects in the photographs became "unreal" in proportion from front to rear because the larger separation gave them too much depth.

Greenslade and Green also found that stereo photographs could be made by rotating the camera through a small angle (they made it 7.7 degrees) between successive photographs. The rotation apparently altered the perspective enough for the viewer to be able to perceive depth when he made the two photographs fuse. You might want to experiment with a range of angles to see if the three-dimensional effect persists and if large angles give rise to distortions in the perceived view.

I had easy success with a Polaroid Model 104, making street-scene stereo photographs holding the camera by hand. Using a tripod to displace or rotate the camera would undoubtedly improve the quality of the pictures, but the three-dimensional illusion was readily apparent even with my casual efforts. I aligned the left side of the camera's field of view on an object and made one photograph. Then I rotated the camera five or 10 degrees to the left, aligned it so that the reference object was at the same height in the field of view and made the second photograph.

Next I trimmed the white edges off the photographs and put them in the stereoscope. As I viewed the pictures through the instrument I slid one of them across my field of view until it approached the correct position, whereupon the photographs suddenly snapped together into a single image with depth. Then I taped the pictures together and mounted them on stiff cardboard to keep them at the right spacing.

When the photographs were correctly positioned, my left eye saw more of the left side of the scene than my right eye did. This difference is automatic if you rotate or laterally displace the camera. Even if I photographed exactly the same scene without moving the camera between successive exposures, I could still get a reasonably good three-dimensional illusion by fiddling with the position of the photographs in my stereoscope. I find, however, that this does not work for everyone.

Making your own stereo cards is great fun. You could even use an old Polaroid snapshot if you have it duplicated, either by Polaroid or by making a photograph of it yourself. For new pairs of photographs the best type of scene to shoot is one with objects in both the foreground and the background.

You normally assign depth to a scene with the aid of several clues, among them the fact that you usually know the relative sizes of the objects you see and so can judge their distance partly by the relative angles they subtend in your field of view. Also of help are shadows and the converging geometry of things such as walls and roads as they are seen receding from you.

You also unconsciously sense the accommodation and convergence of your eyes as you view an object in a scene. Accommodation reflects the amount of muscular stress that must be placed on the crystalline lens of the eye to alter its shape so that the image of the object is in focus on your retina. This clue to the depth does not depend on an interplay between the two eyes: it is monocular.

Convergence is a binocular clue, having to do with the angle between the optical axes of the eyes. The axis is an imaginary line running through the center of the lens and into the fovea, the pitlike depression on the retina where the photoreceptors are the most densely packed. Normal viewing involves moving the eyes to the left or the right until the image of the object falls on the fovea. For a distant object the optical axes from the two eyes are nearly parallel. For a nearby object they are at a significant angle. The muscle control necessary to make the optical axes converge on an object helps you to gauge the distance of the object.

Suppose you view a collection of objects that are at different distances. You will have a sense of their different depths even if you do not look at them individually because the positions of their images on the retina around the fovea will be different for the two eyes simply because of the separation of the eyes. The difference in position with respect to the favea will be greater for a nearby object than for a distant one. Through this effect the brain can assign proper depths to the objects.

In re-creating depth from a two-dimensional photograph the problem is to trick the brain into ignoring the clues that suggest it is seeing a photograph. When you look at a photograph both the focusing of each eye and the convergence between them tell you that you are looking at a photograph rather than a real scene. Two adjacent photographs are no better; your brain knows you are only looking at two adjacent photographs.

The stereoscope enables you to focus on the photographs while you are forcing your eyes to converge: the result is that the images occupy the same positions on the retinas they would occupy if you were viewing the real scene. This clue is so strong, at least for people who can see the three-dimensional illusion, that you are convinced there is depth in the fused pictures. The brain so wants to assign depth that the photographs in the stereoscope do not have to be exactly in their proper position. As they come close to the proper position the brain suddenly snaps their images together and immediately begins to assign depth. It apparently can do this to some degree even with photographs that are exact duplicates.

Although a stereoscope is a convenient means of getting the illusion, it is not strictly necessary. You can see depth in a stereo card alone by means of a simple trick. After focusing your eyes on an object about five feet away quickly slip a stereo card into your field of view. As you are focusing on it, try to maintain the same convergence of your eyes as you had for the object at five feet. With some practice this trick will enable you to merge two images from the stereo card into a single image having depth, just as you do with a stereoscope. I can actually create this three-dimensional illusion at will and without the trick by merely staring at a stereo card while deliberately adjusting the convergence of my eyes until the two images, one from each eye, merge.

With either the trick or the brute force technique what you see is noticeably different from the illusion in the stereoscope. Without the instrument you see, on each side of the image with depth, an additional two-dimensional image of the scene in the stereo card. Greenslade and Green offer a demonstration to explain the two flat images. Focus your eyes on an object about five feet away, simultaneously holding two pencils vertically about 10 inches in front of our eyes and about three inches apart. You will notice that you see not two pencil images but four, two with each eye. As you concentrate on the distant object, move the pencils closer together until the two inner images merge. It is this merging of images that forms the three-dimensional image when you view stereo cards through a stereoscope. With the pencils the two outer images are still present: the left image is the left pencil as seen by the right eye and the right image is the right pencil as seen by the left eye. In the stereoscope those two outer and distracting images are eliminated by the thin wall in the apparatus that is positioned between the eyes and perpendicular to the face.

IN May I discussed the shape of smoke 1 plumes from chimneys under various vertical temperature profiles. G. Frederick Stork of Chevy Chase, Md., has sent me a photograph of a rare occurrence: a smoke ring. Although he did not see the emergence of the ring, he and I believe it must have been blown from the chimney that appears in the foreground. As a puff of hot gas is sent through a circular opening the viscosity between the wall and the gases adjacent to the wall forces those gases to move slower than the gases in the center, causing the latter to curl around the former once they have emerged. The curling creates the ring structure. The stack must have somehow condensed water in the puff and so made the ring visible.

As many readers pointed out, I should make three corrections in my July article about chemical oscillations: (1) The oxidation of a chemical means that electrons are removed and reduction mean that electrons are added, not vice versa (2) A two-normal solution of sulfuric acid is equivalent to a one-molar solution, not the other way around. (3) In adding Tritox X- 100 surfactant or "photoflo" in A. T. Winfree's oscillator us one microgram per liter.

Winfree still has samples of his chemical waves for which you can send. Information on ordering these beautiful and inexpensive samples appears on page 156 of the July issue.

Bibliography

EXPERIMENTS WITH STEREOSCOPIC IMAGES. Thomas B. Greenslade, Jr., and Merritt W. Green III in The Physics Teacher, Vol. 11, No. 4, pages 215221; April, 1973.

PAPER MOITE MACHINES. Budd Wentz. Troubadour Press, 1975.

MAKING VICTORIAN KINETIC TOYS. Philip and Caroline F. Sayer. Taplinger Publishing Co., 1977.

READY-TO-MAKE PHOTO & SCENE MACHINES. Budd Wentz. Troubadour Press, 1977.

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