MANY TECHNIQUES HAVE BEEN employed to make holograms, the intriguing three-dimensional photographs that give a viewer the same sense of parallax and depth that seeing the actual scene would. Recently a new arrangement was sent to me by John Osborne and Bob Waterman of the South London Science Centre. Their technique is the simplest one I have seen It also is quite insensitive to vibrations, which plague other methods of making holograms.

Conventional photographs record the light scattered from an object and focused onto a strip of film. The result is a flat picture with no apparent depth. Holograms are different because they record not the focused image of an object but the interference between the light scattered from the object and some of the direct light from the source illuminating the object.

In the basic procedure for making a hologram light from a laser is split into two beams by a partially reflecting glass plate. One beam results from the transmission of light through the plate. The other beam results from the reflection of light from the surface of the plate. One of the beams directly illuminates a strip of film. The other beam scatters from the object being photographed. Part of the scattered light also illuminates the film, interfering with the direct illumination. The film is thus exposed to the bright and dark components of the interference pattern, which are mostly too fine for the eye to see directly.

Once the film is developed the interference pattern can be reconstructed by placing the film in the beam of a laser. When one views the film illuminated in this way, the interference pattern creates an image of the object photographed. The image is virtual since its existence is illusory, depending strictly on the observer's eyes to focus the rays of light coming from the film. The illusion can be quite convincing because the parallax and depth in the original scene are faithfully preserved. When one examines the film from a slightly different perspective, the eyes lie in a slightly different part of the interference pattern. Therefore one sees a slightly different perspective of the virtual image, just as if the object were actually present.

Making a hologram normally requires several lenses, a partially reflecting plate of glass and a sturdy platform. The

entire arrangement of the laser and the optical components must be as free of vibration as possible while the film is exposed, otherwise the interference pattern will be blurred on the film. Experimenters with holograms often go to elaborate lengths to eliminate vibration from their equipment.

Osborne and Waterman have done away with the need for lenses and glass plates by a clever arrangement of the laser beam and a shiny drop or bead of mercury. The laser is mounted on a tripod so that the beam is directed downward onto the bead of mercury, which is on a small platform on the floor. (The fact that the apparatus occupies little of the floor accounts for the relative lack of vibration.) The object to be photographed is placed near the bead, after which a cylinder of cardboard or plastic is slipped over the entire assembly. On one side of the cylinder, somewhat above the level of the object, is the film to be exposed.

The laser illuminates the mercury bead, which sends part of the light directly to the film and part to the object to be photographed. Some of the light scattering from the object also exposes the film and interferes with the direct illumination on the film. Once the film has been developed it is a hologram of the object photographed.

To see the hologram one mounts the developed picture in an aperture cut into the side of another cylinder. This cylinder is placed around the mercury bead just as the first cylinder was. When the beam again illuminates the mercury, part of the light is reflected to the film. Receiving this light, the observer sees a virtual image of the object lying somewhere inside the cylinder.

One can re-create the illusion with almost any reflecting surface substituted for the bead. For example, a piece of chalk or white paper will reflect enough light to the film to enable the observer to see the virtual image. One can even eliminate the laser by substituting a small white lamp at the point where the mercury bead is normally placed. (The laser is, of course, still needed for making the hologram.) With the lamp the hologram will be colored but not as clear as it is with monochromatic laser light. The various colors from the white light will each create a holographic image slightly shifted in the observer's field of view. Part of the blurring is eliminated if the observer views the image through a colored filter that eliminates all but one color.

For a source of white light Osborne and Waterman use a two-volt flashlamp bulb of the miniature prefocus variety. The bulb has a small filament and therefore serves well as a point source of light to mimic the original source of laser light from the mercury bead. The lens end of the bulb is blackened so that light shines only from the sides.

The helium-neon laser used to make the exposure does not need to be very powerful. The more power it has, however, the shorter the exposure must be and hence the less vibration will be a problem. Osborne and Waterman use a Metrologic laser with a power of no more than .7 milliwatt, which is at the high end of the range of power of the lasers in school laboratories.

The laser is left running during preparations for the exposure of the film, but the beam is blocked by a shutter mechanism from an old camera. When the preparations are complete, the shutter is triggered by a cable in order to avoid jarring the apparatus. A typical shutter speed is .3 second. An index card could be substituted for the shutter, but the exposure would be a bit less reliable.

This technique of making holograms has four disadvantages. One is that the object must be close to the bead and the film and therefore only a clasp photograph can bc made. Second, since the light scatters upward to the object and the film, the arrangement yields an image with a footlight appearance. Third, optimum adjustment of the intensity of the direct and the scattered light is difficult. Increasing the amount of one necessarily means decreasing the amount of the other. Ideally the two should have about the same intensity at the film, so that an interference pattern of good quality is produced. Finally, 11 much of the light reflected by the bead is directed upward and so is lost.

The bead of mercury should be about 1.8 millimeters in diameter. Clean mercury can be extracted from a thermometer

by heat and collected in a small dimple on a thin piece of plastic. When the mercury is not in use, it should be protected from dust to preserve its shine. v The best size of drop can be determined only by experiment. The strategy is to obtain the amount of light at the level of the film that yields the proper interference pattern. A larger bead of mercury can be used to illuminate a developed hologram, since it is no longer necessary to split light off to the object.

The best place for the experiment is a concrete floor in a basement. Upper floors will be subject to more vibration. Some experimenters put their equipment in a sandbox mounted on inner tubes. The large mass of sand decreases the vibrations and the inner tubes help to isolate the experiment from the vibrations in the room. (At the finals of the 1979 International Science and Engineering Fair in San Antonio, Susan Tomlinson of Muskogee, Okla., showed me how she made holograms in a sandbox that was originally a bathtub.)

In addition to damping vibrations one should also prevent convection currents and one's own breath from passing through the cylinder during the preparations. Both of them can create variations in the refractive index of the air in the cylinder. The variations would lead to variations in the interference pattern at the film and so to blurring of the hologram.

Osborne and Waterman prefer working with Agfa-Gevaert 10E75AH film (35 by 31 millimeters), but other kinds can be substituted, including 8E75AH and 20E75AH. The film is developed in Agfa G3p developer for five minutes at a temperature of 20 degrees Celsius. The fixer, which is the usual solution for fixing black-and-white photographs, also requires five minutes.

When the apparatus is set up and one is ready to make a hologram, the first step is to check for the proper illumination at the level of the film. With a piece of translucent paper mounted in place of the film examine the illumination reflected by the bead of mercury and by the object to be photographed. If the illumination is not uniform, try a different size of bead (probably a smaller one) or move the bead around in the laser beam.

When the illumination appears to be optimized, remove the translucent paper, block the laser beam with a shutter or an index card, turn off the room lights, take the film out of its container and mount it in the film holder in the cylinder surrounding the apparatus. After exposure the film is developed, dried and then examined in the setup for viewing the hologram. A successful exposure will be achieved only after some experimentation with the illumination and the shuttering of the laser beam. One should try to get a slight darkening of the film.

The hologram should also record as much of the object as possible. For example, Osborne and Waterman have photographed a small toy goat. When the hologram is viewed, the image of the goat can be turned by turning the cylinder holding the hologram or by moving one's head around the cylinder. One can first see the goat face on, then from the side and finally from the rear.

The laser beam can be dangerous particularly if it reaches the eye directly. Osborne and Waterman take two precautions to eliminate the possibility that the observer might directly view the mercury bead when it is illuminated with laser light. Over the top of the cylinder they place a cardboard sheet with a hole punched in it for the passage of the laser beam. Hence an observer cannot look into the cylinder to see the bright bead. As a second precaution they extend the top out far enough so that the observer also cannot look directly at the bead when looking through the hologram positioned for viewing.

Experimental techniques to demonstrate fluid flow have always been challenging. Usually they employ dye streamers or small tracers of some kind. The techniques are typically clumsy and difficult to interpret, particularly if the flow involves vortexes. If the object being studied is a swimming fish, the techniques are almost impossible to use. How does one induce a fish to remain stationary before a tracer is released and then, on a cue from the experimenter, to move for only a brief amount of time? If multiple markers are employed, the wake left by a fish may be so complicated that the observer will need computer assistance to make sense of it.

A novel system for avoiding these problems has been devised by C. W. McCutchen of the National Institute of Arthritis, Metabolism, and Digestive Diseases. His system provides just enough information for the observer to easily interpret the flow. It does not require messy dyes or small markers. Best of all, a living specimen need not wait for a signal from the experimenter to begin swimming.

The system employs polarized light from point sources to create a shadow of a specimen and its wake. Two lights

illuminate an aquarium containing the specimen. A polarizing filter is mounted on each lamp so that the beams from the lamps are polarized perpendicularly to each other. The strategy is to cast two shadows of the specimen on a screen behind the aquarium. An observer examines the shadows through special polarizing glasses that have the polarization axes of their lenses perpendicular to each other. With the lights and glasses properly oriented the left eye sees only the shadow cast by the lamp at the left and the right eye sees only the shadow cast by the lamp at the right. The brain merges the two images to form what is termed a stereoscopic shadowgram. Although the shadows on the screen are flat, the composite stereoscopic shadow appears to have ordinary parallax and so provides an illusion of depth.

The wake of a moving fish does not cast a normal shadow because water is transparent. To create a shadow of the fluid flow McCutchen begins by arranging for a vertical temperature gradient in the water. When a fish swims through the thermally stratified water, the wake leaves regions with sharp temperature boundaries. Light crossing such a boundary is refracted. As a result the wake throws a shadow on the screen that consists of bright and dark lines. Each lamp makes a shadow of the wake. The visual system merges the two shadows and fills in enough information for the observer to visualize a complete three dimensional wake. This approach does not require the fish to pause for a cue.

A mild temperature gradient (one or two degrees C. per centimeter) suffices for the experiment. The gradient need not be linear but should extend over most of the depth of the water in the aquarium. McCutchen's scheme for a continuous flow of water through the aquarium entails adding hot water at the top while cold water enters at the bottom. Water is removed about halfway up the sides. Sponges are tied over the inputs to keep the fresh water from mixing the water in the aquarium and destroying the temperature gradient. The water does not have to flow, but without flow the experimenter has only a few minutes before mixing destroys the temperature gradient. That, however, may be enough time for a simple experiment or for observing a fish.

For ease of viewing, the observer can mount the polarizing filters and lamps on a helmet. In this way the proper optics are maintained even when the observer moves to get a new perspective. The lamps are six-volt microscope lamps mounted about 10 centimeters apart. Larger separations increase the illusion of depth. Although the observer also sees the true object, it is soon ignored in favor of the shadows. (Because you are working with water, run the lamps from a storage battery that is not connected in any way to the household current.)

The screen must not depolarize the light or both eyes will see both shadows. McCutchen uses the dull side of aluminum foil, mounted with the length of the strip parallel to a line between the viewer's eyes. The arrangement usually provides shadow images of approximately equal intensities, thereby aiding the stereoscopic illusion. Other metals with dull finish, surfaces covered with aluminum paint and ground-glass transmission screens will also serve. McCutchen mounts his aluminum foil over a wood embroidery frame. When the assembly is lowered into the water, the wood swells, stretching the foil tight. Th screen can be either vertical (to facilitate a horizontal view of the specimen) or on the bottom (to give an overhead view of the specimen). Surface waves may distort the shadows in the latter setup. To avoid the problem McCutchen floats sheet of clear plastic on top of the water by means of Styrofoam blocks.

The length of time the wake will persist depends on two factors: the buoyancy forces on the parcels of water in the wake and the rate of heat conduction between the parcels. The buoyancy forces result from the dependence of the water density on temperature. Cold water is denser than warm water and sinks in it. With a temperature gradient of about 1 degree C. per centimeter a wake will last for about 15 seconds. After that the sharp temperature boundaries in the wake are smoothed by the sinking and rising of water and by the exchange of heat between parcels of water at different temperatures.

Many of the problems in classical aerodynamics can be investigated with McCutchen's system. For example, a little wing drawn through water leaves a visible vortex sheet similar to the ideal one described in most textbooks. Vortex rings like smoke rings can be created with the aid of a cylindrical plastic container that has a small circular hole punched in the bottom. Filling the container with hot or cold water and giving it a quick squeeze will form a beautiful vortex ring. With several squirts or with several holes one can have vortex rings that interact with one another.

McCutchen has employed his rig to study the motion of a small fish, a zebra danio (Brachydanio rerio), which is about three centimeters long. The study revealed a complex variety of propulsion mechanisms, none of which appear to be very efficient. Both the main propulsion and the mechanism for changing course come from a vigorous but brief flick of the tail, which is followed by a period of coasting. During the push the spread of the dorsal and anal fins changes subtly. During the first part of the flick all the fins are spread, but on the return stroke the dorsal and anal fins are less spread. When the fish begins to coast after a stroke, all fins are collapsed; they spread out as the fish slows down. A quick stop is usually executed with the pectoral fins, but sometimes the fish puffs a small amount of water out of its mouth to increase the braking effect. When the fish hovers, it maintains its position by fanning water downward with its pectoral fins and tail.

These motions and others are visible in the shadowgrams, but they are too fast for the eye to follow. In order to slow down the motion and also to make a permanent record McCutchen sacrifices the depth in the shadowgram and photographs the shadow cast by a single light. The setup is shown in the illustration [above left]. One large field lens sends a parallel beam of light through the aquarium. Another such lens focuses the light onto a motion-picture camera. The camera is focused on a spot about halfway to the lens in front of it instead of on the aquarium. McCutchen says this focusing increases the sensitivity of the camera to slight inhomogeneities in the refraction of the light passing through a wake. A red filter is inserted into the light so that the camera will not record the chromatic aberration of the field lenses.

The camera is run at about 44 frames per second. Even though McCutchen could slow down the motion, freeze it and reverse it, the fish he studied still moved too quickly for all the details of its motion to be captured. Surprisingly little is known about the propulsion mechanisms of fish, so that if you take up McCutchen's kind of experimentation, you may be able to contribute to that knowledge.

Several people have written to me about my discussion of non-Newtonian fluids in this department for November, 1978. A non-Newtonian fluid is a rather strange substance characterized by the fact that its viscosity changes when the fluid is stressed and sheared. Newtonian fluids, such as water, do not behave this way.

Among my correspondents was I. Slabicky of Newport, R.I., who described how to produce the Kaye effect easily. The effect is a peculiar leap of a stream of viscoelastic fluid when it is poured into a container of the same fluid. Slabicky poured a thin stream (1/16 to 1/8 inch in diameter) of Agree shampoo into his cupped hand from a height of about six inches. By adjusting the height and the width of the stream he discovered a certain combination that produced the Kaye effect almost continuously, giving leaps of up to an inch.

Mary R. Hebrank of Duke University told me about the many important biological fluids that display non-Newtonian features. Within a certain range of shearing rates blood is non-Newtonian, a fact that may be important to some aspects of blood flow through human beings and other animals. Also under investigation are the non-Newtonian aspects of the synovial fluid, which lubricates the joints in vertebrates, and of mucus.

Mucus may play an important role in reducing the drag on some types of fish. According to some theories, this reduction of drag may depend on the non-Newtonian behavior of the mucus (or slime) on the surfaces of the fish. For example, when a fish must swim quickly, some of the slime on its body may be mixed with the passing water. At higher swimming speeds the layer of water near the surface of the fish tends to break up into turbulence, increasing the drag. The addition of a small amount of slime to the turbulent water may increase the viscosity and thereby reduce the turbulence, so that the fish can swim faster.

Many of the demonstrations I described can be seen in a motion picture, Rheological Behavior of Fluids, made by Hershel Markovitz of Carnegie-Mellon University. The film is distributed by the Encyclopaedia Britannica Educational Corporation. The film includes several extremely interesting experiments with dye tracers in a non-Newtonian fluid surrounding a rotating sphere. It also beautifully demonstrates the Weissepberg effect, in which a rotating fluid climbs a central rod rather than the outside wall of the container.

Barbara H. Shafer of Greene, R.I., wrote me about the strange behavior of her butternut squash. She usually cooks the squash before a holiday and then freezes it. As she prepares the squash it shows no signs of being waterlogged, but when it thaws after being taken out of the freezer, a noticeable amount of water collects at the top of the container. She heats the squash in a double boiler, but even then the water is not reabsorbed by the squash. Therefore just before serving the squash she stirs it, and the more vigorously she stirs the thicker it gets. The shearing effect of the stirring apparently increases the viscosity of the stuff. Shafer doubts that all squashes behave this way, so that if you want to repeat her experiment, you could try different types of squash.

Bibliography

FLOW VISUALIZATION WITH STEREO SHADOWGRAPHS OF STRATIFIED FLUID. C w. McCutchen in The Journal of Experimental Biology, Vol. 65, No. 1, pages 11-20; August, 1976.

FLUID DYNAMIC PHENOMENA CAN BE DEMONSTRATED WITH STEREO SHA OWGRAPHS OF STRATIFTED FLUID. c. w. McCutchen in American Journal of Physics, Vol. 44, No. 10, pages 981-983; October, 1976.

FROUDE PROPULSIVE EFFICIENCY OF A SMALL FISH, MEASURED BY WAKE VISUALISATION. c. w. McCutchen in Scale Effects in Animal Locomotion, edited by T. J. Pedley. Academic Press, 1977.

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