MOST AMATEURS who make photographs of astronomical objects use color film. Color emulsions record more information than black-and-white film does and cost only a little more. Not all amateurs who make such photographs extract the maximum information from their color slides, however. Subtle differences in hue can be easily overlooked. John Sanford (2215 Martha Avenue, Orange, Calif., 92667) has hit on a photographic technique for enhancing those subtleties. He makes black-and-white prints of the color transparencies with both ordinary and panchromatic projection papers exposed through certain color filters. Differences in color that would ordinarily escape detection in a color print stand out as patterns of strong contrast in the black-and-white prints.

"A great deal of information frequently escapes detection in color slides made of the night sky," Sanford writes. "Much of it, however, can be recovered in the darkroom with black-and-white photographic paper and a few filters. Essentially a black-and-white projection print is made of the color slide. The resulting print is a negative image of the slide.

"The slide is placed in the enlarger to expose the black-and-white paper to make a paper negative on which stars appear as dark spots against a white background. Many laymen have seen photographs of this type; typical are the prints from the Palomar Sky Survey that appear occasionally in magazines. Professional astronomers prefer reproductions of this kind because of the ease of marking objects (for duplicating the photographs with office copiers) and because faint detail on the original plate appears on the negative print as distinct regions of gray or black against the contrasting white background of the sky.

"Certain slide films have a high sensitivity to red light. They are excellently suited for recording the reddish light from H II (ionized hydrogen) regions of the galaxy, where one finds most of the well-known diffuse or galactic nebulas including M42 (the Great Nebula in Orion), M8 (the Lagoon Nebula) and NGC 7000 (the North American Nebula). My color slides have been made with Fujichrome 100, which is available in most camera stores. This film responds well during processing to 'pushing,' which is controlled overdevelopment to compensate for underexposure. The film can be developed with the standard E-4 process, as Ektachrome is, either by a commercial processor or by the photographer himself with a kit. My films are processed by a commercial laboratory. I usually specify a one-stop push to ASA 200. A 10-minute exposure with Fujichrome 100 is equivalent in my experience to a 30-minute exposure with High Speed Ektachrome, assuming that both films are pushed one stop during processing.

"The projection prints are made on either regular or panchromatic paper, depending on whether the observer wants to enhance the blue parts of the spectrum or the red. Blue hues can be enhanced by making the print with regular enlarging paper, which is sensitive to blue-green light. This selectivity explains why the darkroom can be illuminated with a yellowish safelight. Regular enlarging paper is even less sensitive to red light.

"Blue hues can also be enhanced by exposing panchromatic projection paper through a blue filter, such as a Wratten 38A. This filter is available inexpensively in the form of gelatin film at Eastman Kodak stores. Expose the print just to the point at which the background begins to take on a perceptible tone of gray. That exposure will maximize the detail of nebulas. The time of exposure may seem overlong to anyone accustomed to making enlargements of conventional black-and-white photographs Be patient. To enhance reddish detail use panchromatic projection paper, such as Eastman's Panalure, and expose it through a Wratten 23 or 25 filter.

"A photograph of the center of Scorpius made by a 20-minute exposure of Fujichrome 100 color film with a 105millimeter Nikkor lens at f/3 apparently failed to disclose differences in the density of dust lanes that characterize this region of space. The lanes do show clearly, however, in the enhanced negative print of the slide [see illustration above]. A similar enhancement of blue hues disclosed many details of the North American Nebula that are barely perceptible to the eye when the E color slide is examined [see illustration below]."

Casual users of reflecting telescopes rarely fit their instrument with a setting circle or any other elaborate device for centering the eyepiece on an object. Instead they take aim with a gunsight or with crossed wires in a short length of tubing. These expedients require the observer to cock his head at awkward angles when he wants to scan certain parts of the sky. A more efficient and convenient accessory, an elbow finder telescope, has been improvised by Harry F. Kale (1800 Galaxy Drive, Newport Beach, Calif. 92660). He describes the device as follows.

"My finder consists of an eight-inch right-angle tube three inches in diameter. Between thin metal rings at its ends it supports a pair of three-inch magnifying glasses of four-inch focal length. I bought the magnifying glasses for $1 each.

"Two tin cans from which the ends have been removed can be soldered together to form the tube. Before I assemble the tube one can is cut in half at an angle of 45 degrees, rotated and resoldered to make a right-angle elbow. The elbow section is then soldered to the other can.

"A cylindrical block of wood cut to form a 90-degree prism was cemented inside the elbow to support a front-surface mirror at an angle of 45 degrees [see illustration at left]. Two lengths of No. 18 stovepipe wire were threaded at right angles through the tube four inches from the magnifying glass that functions as the objective lens. In this position the crossed wires are sharply focused by the lens that functions as the eyepiece.

"A front-surface mirror is essential as the diagonal element for bending the light rays to avoid multiple reflections and hence multiple star images. I bought the mirror for $3 from a mail-order house that sells optical supplies. I reshaped the mirror with a grinding wheel to fit inside the elbow. The mirror was fixed to the supporting prism by silicone-rubber cement.

"Support rings for adjustably mounting the finder telescope to the main telescope were cut from a discarded aluminum ashtray. Three equally spaced holes were drilled in each ring and threaded for radial adjustment screws that clamp the finder. A single bolt attaches each ring to the tube of the main telescope at positions where the optical axes of the two instruments are approximately parallel. Final alignment can be made during the daylight hours by first centering a distant object in the eyepiece of the main telescope and then centering the cross hairs of the finder on the same object by manipulating the adjustment screws of the aluminum rings. Although the finder does not magnify the star field, the light-gathering power of the three-inch objective lens significantly increases the apparent brightness of all celestial objects. The cross hairs can be seen clearly in silhouette against the background of stars, and the elbow scheme greatly increases the comfort of examining the night sky."

D. G. PRINZ (37 Parkville Road, Manchester M20 9TX, England) recently noticed that the reflected image of a drafting instrument made of clear plastic appeared highly colored in a shaft of sunlight that had bounced off the painted side of a filing cabinet. The observation started Prinz, who is a computer consultant, on an investigation that led him to the report of a similar observation made late in the 18th century by Etienne Malus, an engineer in the French army. Malus had seen the same colors when he looked through a calcite crystal at light reflected from the windows of the Luxembourg palace. He explained the effect as the consequence of polarization, that is, the rotation of the direction in space in which light waves of differing color vibrate.

The same effect had subsequently attracted the interest of the Scottish physicist Sir David Brewster. Brewster found that maximum polarization occurs in light reflected from a substance at an angle with a trigonometric tangent numerically equal to the index of refraction of the substance. For example, light reflected at an angle of 53 degrees 7 minutes 46 seconds from water is more strongly polarized than light reflected at greater or lesser angles. The trigonometric tangent of this angle is 1.3333, which is also the index of refraction of water at the temperature of 20 degrees Celsius. (The index of reaction expresses the ratio of the speed of light in a vacuum to its speed in water.)

The index of refraction of most ordinary plastics is approximately 1.45. Prinz reasoned that light reflected from the polished surface of a plastic should be strongly polarized at the angle (about 55 degrees 24 minutes of arc) that corresponds to the trigonometric tangent of 1.45. To check his conclusion he hinged together with flexible adhesive tape two sheets of polished black plastic, much as the covers of a book are joined. He laid the assembly in sunlight on a windowsill with the hinge facing into the room. When he raised the upper sheet so that it made an angle of about 110 degrees with its mate, its inner surface was flooded with sunlight reflected by the lower sheet.

Prinz hinged between the sheets of plastic a few pieces of transparent cellophane. He held them so that they would intercept the polarized rays. The cellophane displayed dazzling colors.

Cellophane, which consists of reconstituted cellulose, has the property of birefringence. It splits incident light into two components that propagate at different speeds, which vary with the length of the light waves. As a result some waves of polarized light interfere destructively in birefringent substances,

depending on the thickness and the orientation of the substances. Such waves are not seen by the observer.

The remaining light appears as the complementary color. The hue that is seen depends on the thickness of the cellophane and its orientation with respect to the plane in which the polarized light vibrates. Prinz kept a record of the number of sheets of cellophane and the orientation required to generate each color. The data enabled him to employ the cellophane to create full-color pictures that resemble miniature stained-glass windows.

The polarizing sheets need not be made of polished black plastic. Prinz achieves the same effect by blackening white business cards with India ink and, when the ink dries, making them reflective by covering the surface with shiny transparent adhesive tape. A coating of quick-drying acrylic varnish can be substituted for the tape. Images reflected from such surfaces may appear grossly distorted because strips of plastic tape on cardboard do not make an optically true mirror. Nonetheless, vivid colors appear. The blackened material merely improves the contrast in the image.

FREDERICK S. Duncan (P.O. Box 212, Pima, Ariz. 85543) reports a dramatic experience that in 1969 introduced him to a wildly esoteric field of art. Duncan is a systems engineer and was stationed at Thule in Greenland at the time. "For two years in Thule," he writes, "I whiled away the long winter nights by trying, among other things, to find all the solutions to the tangency problem of Apollonius of Perga, that is, to draw a fourth circle so that it just touches each of three given circles. Another of my diversions was experimenting in a similar way with a Steiner chain. In this chain each circle of a finite number of circles must touch two other circles in the series as well as two that are not part of the series.

"Once I happened to rotate one of my sketches about an axis perpendicular to the paper, hoping thereby to find a point of view that would clarify the problem. Suddenly I became aware of an effect that made me feel like an inhabitant of Flatland viewing his village and its inhabitants for the first time from a balloon. Some of the circles rose out of my sketch and floated in the air! Others sank below the surface. Their positions interchanged again and again as I watched. I could only conclude that my senses were reflecting our overlong stay in Greenland.

"I later learned that the effects had been investigated in a preliminary way half a century ago by two Italian psychologists, V. Benussi and C. L. Musatti. Benussi pointed out that when a circle is drawn as a line of constant thickness on a disk, it will appear motionless if the disk is rotated about a center that coincides with the center of the circle. Displacing the center of the circle from the center of rotation creates the illusion that the circle is rising above or sinking below the surface of the disk on which it actually rests. The effect can be enhanced by closing one eye.

"Replacing the circle with an ellipse further confounds the eye. The pattern may revert to a circle that gyrates wildly in three dimensions. It may warp and stretch with an amoeboid motion. These phenomena are known as Musatti effects.

"After my return to the U.S. in 1971 I made a number of designs in color: bright reds, whites, blues, greens and blacks. The patterns were drawn on heavy disks of cardboard 18 inches in diameter. The disks can be rotated by a variable-speed turntable at rates between two and 30 revolutions per minute. I regard the designs [see illustration at left] as a kind of kinetic 'op' art that gives aesthetic satisfaction to the viewer and that may have interest for those who investigate the mechanisms of visual perception. The patterns are not difficult to draw. An adequate turntable can be improvised from the mechanism of a discarded phonograph. For people who prefer just to look, kits of 12-inch disks in color complete with a motor are available from Research Media, Inc. (4 Midland Avenue, Hicksville, N.Y. 11801)."

DURING a mathematical investigation of electromagnetic forces that arise locally in various electric circuits, Harry E. Stockman (75 Gray Street, Arlington, Mass. 02174) has discovered a number of novel motors. Some of them, such as the tunnel-diode motor and the parametric motor, have been described in these columns [see "The Amateur Scientist," October, 1965, and January, 1973] Stockman's most recent construction is a variable-speed motor that operates on alternating current. Unlike most variable-speed motors, his version requires no commutator or brushes. Stockman describes the motor.

"A demonstration model of the asynchronous motor includes a fixed electromagnet energized by a potential of about 20 volts at a frequency of 60 hertz. The speed of the motor can be altered by varying the applied potential. The moving element consists of a nonmagnetic rotor that supports at one end a coil of insulated wire connected to a silicon diode. The rotor is balanced mechanically by a counterweight and turns on a vertical shaft, as shown in the accompanying illustration [right].

"A bar magnet is supported near the electromagnet. As the small coil approaches the electromagnet it cuts through the field of the magnet at approximately right angles. The design is not critical. All dimensions can be altered."

An alternating potential applied to the electromagnet causes a force of magnetic repulsion to act on the small coil. Simultaneously the alternating potential induces a unidirectional current in the small coil connected to the rectifying diode. The resulting unidirectional magnetic field of the small coil can interact with the field of a bar magnet to minimize the repulsive force exerted on the coil by the electromagnet.

"By experiment one can find a position of the bar magnet that enables the small coil to approach the electromagnet and move freely into alignment with it. At this position, however, the coil is violently repelled. As the coil recedes from the electromagnet it acquires angular momentum that carries it into the field of the bar magnet in preparation for the next revolution The speed of rotation is a function of the forces and hence of the applied potential."

Bibliography

HALF-HOURS WITH GREAT SCIENTISTS. Charles G. Fraser. Reinhold Publishing Corporation, 1948.

SATURDAY SCIENCE. Edited by Andrew Bluemle. E. P. Dutton Company, 1960.

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