"My first rule," he writes, "is to shun the temptation to make permanent slides. They are handy for reference, of course, but I suspect that most permanent collections accumulated by amateurs (including mine) owe their existence largely to our simian love of bright trinkets. You can learn more about protozoa by studying live material than by collecting desiccated specimens.

"Here is one way of preparing a live slide that is the essence of simplicity. Put a drop of culture on a clean slide,: ring it with a little dike of vaseline applied by the tip of a toothpick and drop a cover glass on top. The vaseline retards evaporation around the edge of the cover glass and also prevents it from crushing the animals. Don't worry about smothering them. Long after fatigue has dimmed your eye, the protozoa will be darting about, feeding and reproducing. When you finish observing, return the organisms to a storage jar and wipe the slide clean.

"It is possible to make a lot more work out of this simple operation. You can, for example, purchase slides with circular concavities for the organisms, and you can make elaborate watertight seals to cover them. Some workers spend hours building little ponds on flat slides with rings of plastic or waterproofed cardboard. Others make cells of special cements involving rubber, gutta-percha, shellac and so on. Although many of these slides are pretty and display astonishing craftsmanship, the effort expended does not show up in noticeably better results when you look through the eyepiece.

"It is also easy to make hard work out of the preparation of a permanent slide. Back in the days when slides took the form of machined blocks of ivory fitted with mica windows held in place by slip rings, a proficient worker was lucky if he finished one per week. Some amateur microscopists still get fun out o£ making replicas of these historic slides as well as the time-consuming glass types.

"Anyone who wishes to save time in making a permanent slide will appreciate the method devised about 25 years ago by a German biologist named Bresslau. It is especially suited for mounting ciliated protozoa, because they have relatively tough hides.

"A culture of paramecia makes good practice material to work with while you are mastering the technique. This minute creature looks somewhat like a transparent football covered with a fuzz of short hair. You can cultivate paramecia from timothy hay. Pack a quart stewpan about half full of hay and add enough water to cover it. Boil the preparation for 10 minutes. This kills many of the fungi, spores and other unwanted organisms in the hay. Then discard the water, let the hay drain for a few minutes and transfer it to a clean pint jar filled with either pond water or distilled water. After the infusion has incubated for a week at room temperature, the paramecia cysts will have developed and you will find the jar swarming with organisms. Some workers increase the yield by adding a nutrient, e.g., a generous pinch of powdered skim milk.

"Place a drop of the culture on a clean slide. To stain it, make a concentrated solution of opal blue in water, boil a half teaspoon of it in a small test tube and place one drop of this boiling hot stain on a slide near a drop of the culture. Mix the two drops quickly and smear out lightly with a toothpick. Avoid pressure, for it may crush the larger paramecia. The staining process is completed in about five minutes. On a very dry day it may be necessary to put the slide into a moist chamber to prevent the stain from evaporating before it has had time to act. When the staining is complete, let the slide dry completely. Then add a drop of Canada balsam and gently press the cover glass into place.

"The procedure is not half as complicated as it sounds, and the whole job should not take more than 10 minutes. If it is done correctly, most of the paramecia will show no shrinkage or distortion. Speed, and care in applying pressure, are the only critical elements in the process. I have eight-year-old slides of paramecia [see photographs above] which show no deterioration."

The Foucault test for determining the shape of a concave mirror, capable of accuracy to a millionth of an inch, is the essence of simplicity. You make a pinhole in a tin can, put a candle inside and shine the rays from the pinhole (a synthetic "star") on the mirror. If the mirror has the figure of a true sphere, the reflected rays converge to form an image of the pinhole. When the mirror is viewed from a point just behind the image, it appears evenly illuminated and flat, like the disk of the full moon. And if you pass a knife-edge through the center of the image, the mirror should darken uniformly.

That is the way the test is supposed to work. In practice it is much more interesting-or exasperating, depending upon your temperament. The slightest departure of the mirror from a true sphere-or an equivalent change in its position or an abrupt variation in the density of the surrounding air-destroys the apparent flatness of the disk. With appropriate modifications of the light source, you can take advantage of this sensitive property and use the apparatus for photographing rifle bullets in flight complete with the shock waves. Similarly, you can photograph sound waves convection currents, streamlines around airfoils and so on. The setup can even be adapted as an ultrasensitive seismometer, which will pick up the vibrations of traffic miles away.

Amateur telescope makers have a lot of fun doing experiments like these. But primarily they use the test as the French physicist Jean Foucault intended it: for determining when the mirror has been polished to the figure of a parabola, the shape required for a good reflecting telescope. When the parabola is examined at the knife-edge, it presents a pattern shaped somewhat like a doughnut instead of a flat disk. As the ratio of the focal length to the diameter of the mirror is increased, the distinction between the two patterns tends to disappear. The doughnut becomes flatter with increasing focal length. At about f/15 the curve of the parabola coincides with that of the sphere for all practical considerations, and the Foucault pattern for both appears flat. Below f/5 the "doughnut" develops such pronounced shadows that interpretation becomes difficult and the test loses its usefulness.

Many amateurs have dreamed up schemes for making the Foucault apparatus less finicky. The trouble stems from the fact that, as conceived by Foucault, the pinhole must be situated somewhat off the optical axis of the mirror, so that the reflected image will form on the opposite side, where there is space for the amateur's eyeball. The strongly shadowed doughnut of short-focus mirrors results from this angular illumination.

Carl Bergmark, an amateur telescope maker of San Francisco, submits a solution he has devised for the problem:

"One of the defects of the Foucault test when working with short-focus mirrors is the error introduced by the lateral distance between the pinhole and the knife-edge. I believe that my test rig eliminates this shortcoming [see drawings]. The light from the pinhole is reflected into the axis by a microscope cover glass instead of being directed straight toward the mirror. This arrangement causes some loss of illumination, but the comparatively great light-gathering power of the short-focus mirror compensates for it. Rays reflected from the mirror, as shown by the upper drawing, are transmitted to the eye through the cover glass, the knife-edge being inserted at the point of convergence between them. Although the surfaces of the cover glass reflect a double image of the pinhole, the glass is so thin that the two images may be regarded as a point. I am lucky in having access to a nine-inch metal lathe, the compound rest of which serves as a handy carriage for the gadget. While under test the mirror is supported on a wall bracket behind the lathe."

After making the drawings to illustrate Bergmark's rig, Roger Hayward, who is an old hand at devising optical tricks, observed: "I have a feeling that Bergmark's scheme of using a microscope cover glass for an image divider is not altogether new, but I cannot remember having seen it applied in just this way. The secret of its success lies in the fact that, bad as cover glasses are optically, a very small area of one is always good enough. Incidentally, the formula for computing the difference between the position of the knife-edge at which the center of the mirror appears to darken uniformly and that at which any radial zone of a parabola similarly darkens is computed by the equation: D = r²/2R, where r is the radius of the zone, R the radius of the mirror's curvature and D the difference in position through which the test rig must be moved to achieve the desired darkening."

For some time amateur telescope makers have been wondering how to make use of the war-surplus lenses of wide aperture and short focal length which have been much advertised at prices of less than $20. Many are cemented achromats of superb quality-corrected for two colors and manufactured to rigid tolerances. A number are coated for maximum light transmission and come ready-mounted in precision cells. The catch in using these lenses as telescope objectives, of course, is that their focal length is only about 10 to 20 inches and their magnification is correspondingly low. A one-inch eyepiece gives a magnification equal to the focal length of the objective lens in inches. A half-inch eyepiece doubles this power; a quarter-inch quadruples it and so on. To utilize the maximum light which the surplus lenses are capable of gathering, one would need an eyepiece only an eighth of an inch or less in diameter, and so small an objective is impracticable.

Wilfrid T. Patterson, an optician of Guelph, Ontario, suggests an ingenious if unconventional solution for the problem. "I rather doubt that my idea is new," he writes, "but I can assure you that it works. Some time ago I obtained an f/5 photographic lens of four inches diameter. It was assembled as the objective of a refracting telescope fitted with an Erfle eyepiece taken from what appeared to have been part of a bomb sight. The combination proved excellent for scenic viewing and bird study. The image was inverted, of course, but years of playing with astronomical telescopes: had accustomed me to looking upside down. While working with this instrument, I got an urge to try out some of the surplus objectives. They turned out to be of substantially higher quality than the camera lens. The images were much more brilliant and showed astonishing resolution. It was obvious that they could withstand high magnification without becoming fuzzy. Why not examine them under a compound microscope?

"The idea was given a try by means of a hybrid arrangement using a 40-power microscope of the type designed for machine shops. This gave the completed instrument an over-all power of 400 diameters. The field of view was exceptionally wide and sharp to the very edge. Almost the whole face of the moon appeared in exquisite detail. The objective did not gather quite enough light at this magnification for a satisfactory view of Saturn.

"I next assembled from surplus parts a microscope which could be varied through a wide range of powers. The objective lenses were selected from an assortment of achromats picked up at a few cents apiece. They work nicely as low-power elements and, incidentally, can also be used for building excellent eyepieces. Various tubes designed for spotting scopes and tank sights also are available on the surplus market. One end of these tubes is usually machined to take eyepieces, and the other can easily be modified as a cell for the objectives. With a selection of three or four objectives and eyepieces you have a microscope, ranging in power from 10 to 100 diameters, which will find endless use around your shop as well as in the telescope.

"Alignment of the optical train is critical. The holes in the focusing sleeve and guide flange must be carefully centered. The body of my scope is made of seamless aluminum tubing. It is stocked by dealers in surplus optical parts and is easily worked with hand tools. Hayward, when he made the drawing of the instrument, observed that by using an arrangement in which a thick tube must slide in a solid brass sleeve I am asking for trouble. The combination may stick and even freeze. A springy sleeve would be preferable. The problem was solved in old microscopes by lining the sleeve with velvet-and a fine solution it is."

Patterson equips his astronomical telescopes with slow-motion drives using homemade worm gears. "These drives,' he says, "have many applications. The are handy for adjusting the position o remote optical parts, shifting the frequency of electronic oscillators, driving the charts of pen recorders and so on. With the fixture shown here [drawings left] you can turn out a $20 gear for $1.50. The teeth of the gear are cut by means of a tap, and those of its companion worm, by a matching die.

"I cut the blank gear from brass plate, usually a quarter of an inch thick. The approximate radius of the disk is found by dividing the gear ratio desired by 6.28 (2_) times the number of threads per inch which the tap cuts. If the desired gear ratio is 365 to 1, for example, and you use a tap which cuts 16 threads per inch, the blank should have a radius of approximately 3.66 inches. In practice you make it just a trifle larger to allow for the depth of the threads.

"A hole is drilled in the center of the blank disk so it will turn freely on the stud of the fixture. The blank is advanced by means of the feed screw of the fixture and the tap is rotated lightly through one revolution of the blank to mark the points at which the teeth will be cut. The work is then examined to as sure that the tooth spacing will come out even. If the blank has been made oversize, the spacing between the first and last tooth will be too wide. The blank is then dressed down and another experimental cut is taken.

"There is nothing hard and fast about the design or dimensions of the tooth-cutting fixture. It would be simple, for example, to drive the tap with an electric drill instead of the arrangement shown. But that would hurry the job. After you have spent two evenings or more making the fixture, you will feel cheated unless you have the fun of turning the crank for half an hour or so."

Despite the storms and floods in New England this fall, more than 200 amateur astronomers and a generous sprinkling of professionals turned out for the annual Stellafane meeting at Springfield, Vt.

James W. Gagan, secretary of the Amateur Telescope Makers of Boston and one of the meeting's moving spirits, re-: ports that the weather cleared to 96 per cent seeing and held good for two nights. "Many of the gang," he writes, "camped on Breezy Hill and enjoyed naked-eye observation of clusters and nebulae. Arcturus and the Alcor-Mizar system, along with a comet about nine degrees below Polaris, provided excellent objects for judging the resolution of telescopes placed in competition.

"Winfred Lurcott of Cranford, N. J., who spent two days behind the wheel of his car detouring New England's flooded areas, arrived just in time to set up his big portable reflector and take both first prizes. His 10-inch Newtonian has a rigid yoke mounting, a clock drive, a rotating multiple-eyepiece turret and accurate setting circles [see Figure 4]. It can be stowed in the trunk of a car, and can be unlimbered and set up ready for observation in five minutes flat.

"Other awards were taken by John E. Welch of Springfield, Mass.; Guy Gordon of Natick, Mass.; John Sanford of Newburgh, N. Y.; George Random of West Acton, Mass., and Bruce Woodward of Rye, N. Y. Incidentally, the committee is still trying to locate unidentified winners. The judges tried out a system of tagging entries by number to eliminate any possibility of bias. It worked so well that two contestants left for home without their awards.

"August 11, 1956, has been set as the date of the next Stellafane meeting, that being the first Saturday after the new moon in August."

Bibliography

AMATEUR TELESCOPE MAKING. Edited by Albert G. Ingalls. Scientific American, Inc., 1952.

AMATEUR TELESCOPE MAKING-ADVANCED. Edited by Albert G. Ingalls. Scientific American, Inc., 1954.

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-401-823-7800

Internet: http://www.sas.org/