"If one examines a diagram of the optical system of a submarine periscope, the impression is one of a large array of special lenses and prisms that, needless to say, are expensive and difficult to get. Moreover, the assembly and adjustment of such a system in a watertight tube present many problems that, even if not insurmountable, are difficult enough to discourage amateur effort.

"The lens system of a periscope can be regarded as two telescopes in series pointing toward each other. If the telescopes are of the same power, objects are seen in their normal size. In effect the optical system moves the eye to a position nearer the far end of the tube, and a proportionately larger field of view is observed in comparison with the field that could be seen through a simple tube (one without lenses) of the same length. The prism or mirror for bending the light at a right angle can be either inside or outside the lens system.

"While the above is a qualitative description of a periscope, it does not answer the questions of what focal length and of what quality the lenses should be. One might hazard a guess that if the magnification of the two telescopes comprising the system was fairly low (not over four times), the quality of the lenses would not need to be very high.

"The most inexpensive lenses I could locate turned out to have a diameter of 1 1/4 inches and a focal length of 7 1/2 inches. They were bought at a local variety store for five cents each. Their optical quality proved to be satisfactory, and I found that a remarkably good low-power telescope could be made by combining several lenses for the eyepiece and using a single one for the objective. This trick of changing the focal length of a lens by putting several lenses together is well enough known to those engaged in professional optics, but as an amateur I had to discover it for myself.

"Thus relieved of the problem of obtaining a variety of lenses, the construction of a periscope of the sort that I wished to investigate became practical. My first impulse was to buy for the body of the periscope an aluminum tube that matched the diameter of the lenses and to equip each end with some sort of smooth-fitting ell. Such tubing was not readily available; further study indicated that the construction would present problems of machining that I was not equipped to solve. The most straightforward construction I could think of was a simple box 1 1/4 inches square (inside dimensions) by 50 inches long made of quarter-inch plywood.

"Three sides of the box were first glued together to form a U-shaped trough into which the lenses would fit snugly. The box was made several inches longer than the anticipated length of the periscope to allow for adjustment of the lenses. Leaving the ends of the box open greatly simplified the adjustment. A mirror was used for bending the light at one end. It was inserted outside the lens system after the lenses were adjusted.

"While the assumption is reasonably correct that a periscope is two telescopes in series, a marked improvement in performance can be made by the insertion of additional lenses. After some experimentation, I hit on this final arrangement: Three of the lenses were combined to make the objective and were placed two inches from the end of the trough, then two lenses were combined and placed at the focal point of the objective (about 2 1/2 inches farther down the trough). These served as a condensing lens to conserve light. Then a single lens was placed 7 1/2 inches down the trough from the condensing lens. The seventh lens was placed 15 inches from the sixth lens. This distance is not critical for the center of the field, where the light is parallel to the axis. If the spacing is made much greater than 15 inches, however, increased vignetting, or loss of light at the edges of the field, will be apparent. The eighth and ninth lenses, which form a doublet, were placed at the focal point of the seventh lens, as shown in the accompanying drawing [Figure 1]. This is the center of the symmetrical optical train. To complete the system the remaining lenses are installed in reverse order.

"The focal length of the lenses was determined by supporting a lens in a cardboard bracket and bringing the image of a distant object (the sun) to focus on a cardboard screen. The focal length is equal to the distance between the center of the lens and the screen. The lenses had been made up into magnifying glasses and were supported in a black plastic ring. The handle, attached to this rim, was easily broken off. The rims were left in place, however, to steady the lenses in the tube. In the absence of such a rim one could cut a ring from cardboard tubing and cement the lens to it. The interior of the trough should be painted flat black to minimize internal reflections.

"The trough was placed on a table in a darkened room and pointed at a source of light some 20 feet away. The lenses were placed in the trough in their approximate positions. A small piece of white paper was inserted into the trough at a right angle to the optical axis and moved back and forth. The lenses were then adjusted one by one, starting at one end and working down the trough. Where the light is converging into a lens, an image will be formed, and the lens should be adjusted to this position. Where the light is diverging or parallel entering a lens, no image will be formed, and the adjustment is made by moving the piece of paper to the next focal point and adjusting the intermediate lenses.

"After adjustment the lenses were glued in place with a few drops of quick-drying cement. The fourth side of the trough was then glued on, completing the tube. The end window was cut and the 45-degree mirror inserted. The tube was cut off to a suitable length. One end was closed by a plain square of plywood and the other by a similar piece fitted with a small piece of Lucite. A Lucite window was also provided over the windows in front of the mirror. Finally the joints of the box were caulked with wood filler and the wooden parts given several coats of waterproof paint.

"Some sort of eye-shield should be attached to the upper end of the instrument to aid in keeping the eye centered at a correct distance from the first lens. The shield also minimizes external light and thus enables the eye to adjust to the relatively weak light that comes through the periscope.

"The completed periscope is about 43 inches long and the field of view is 20 degrees. The system includes 16 lenses. The light loss is accordingly high: 5 per cent per surface is the figure commonly quoted, or a total loss of about 60 per cent. This is not fatal, however, and on a bright day is not particularly noticeable after one waits for a few seconds for the eye to compensate. In clear water such as a mountain lake's the range of vision is about 30 feet. (It is possible to make out a four-inch white post at this distance.) If the water contains suspended matter, the range is considerably reduced. The turbidity of the water limits the range more than light intensity.

"The periscope has advantages other than keeping the user dry. It is especially suited for observing in small, shallow bodies of water where a swimmer would disturb the environment. One of the most beautiful places I have found is the depths of a lily pond high in the mountains. The light filtering through the green leaves, the strangely shaped white roots and the small fish and insects present not only a fascinating cross section of marine biology but an unforgettable picture-in natural color."

"The pressure that can ultimately; be reached by a pumping system," he writes, "and the speed at which this pressure is attained are determined by three factors: the volume of the system, the quantity of gas leaked or evolved into the system and the effective speed of the pumping system. Since it is obviously easier to evacuate a small container than a big one, the volume of the vacuum system and experimental chamber should be kept reasonably small. The leak situation can be divided into two parts, the first being leaks as we normally think of them, that is, holes in the system These leaks are apt to be extremely small and hard to detect by normal methods. As an example, a leak that can just barely be detected by submerging the object in water and maintaining pressure inside so that air bubbles will appear at the leak would be regarded as enormous in a vacuum system, and the equipment would probably be inoperable.

"The second type of leak is the 'virtual leak' and is due to the continual evolution of gas from materials contained within the system. This can be corrected by the proper choice of materials, cleanliness and operating procedures. Since a vacuum system is limited by the speed at which a gas can be removed, the effective pumping speed is of great concern. The word 'effective' is important because we are interested in the pumping speed at the chamber to be evacuated and not the pump's rated speed. One of the faults I have noticed in earlier articles presented in 'The Amateur Scientist' was the use of long, small-diameter tubing to connect the pump to the experimental chamber. This tubing presents a resistance to the flow of gas molecules that varies directly with the tube length and inversely with the cube of the tube diameter. It is most important to use short, large-diameter tubing between components.

"The traditional material for vacuum systems is glass. For the average amateur, however, this material makes construction very difficult. The professional often uses two alternate building materials-copper and stainless steel. These materials are not only easier to handle but do not shatter. Generally the choice between these two will depend on the facilities available. Stainless steel is usually welded, and copper is usual brazed or soldered. Copper offers the additional advantage of availability to the experimenter. A good, dependable vacuum system is easily constructed from standard copper tubing (usually the rigid type), and solder fittings are available at any plumbing supply house. The tubing is cut to length, and the ends are thoroughly cleaned with steel wool. Flux is then painted on the area to be soldered. The copper fittings are similarly prepared, the tubing is inserted and is carefully seated in preparation for soldering. All joints must be soldered carefully so that the solder flows evenly through the entire joint. We usually use hard (silver) solder and a borax-base flux; however, a fluxless type of solder or high-tin-content soft solder can be used with the understanding that the ultimate pressure, speed and highest temperature that can be reached will be limited by the type of solder and flux used. After applying flux to the area to be soldered, the torch (either household gas and air or propane) is played on the fitting and tubing to heat the general area thoroughly. As the flux bubbles and fluffs up the torch is gradually moved closer to the area of the joint. The solder wire is frequently touched to the joint to determine when the work is hot enough for soldering. Solder is then fed into the joint (usually from the outside) while the torch is directed onto the fitting.

"The solder should flow freely into the fitting. Since it will follow the heat, it is possible to pull the solder completely through the joint merely by directing the heat from the torch to the opposite side. As soon as the solder has solidified the hot joint is plunged under running water to break off the flux. Soldering technique is most important, not only because poor solder joints are susceptible to leaks but also because such leaks are exceptionally difficult to find. A good solder joint will show a smooth, even fillet on both sides, indicating that solder has run completely through the joint.

"Most of the vacuum system can be built of copper tubing. This construction, is recommended. Two problems still exist for the amateur. The first is that of temporary or semipermanent joints. These can be made of flanges that can be purchased. Flanges are sealed by rubber or Teflon 'O' rings. The manufacturers of O rings will also usually supply flange design data from which the amateur can make flanges to meet his needs. We have found from experience that the machining of a 45-degree bevel about 1/8 inch deep around the hole into which the tubing is soldered will ease the job of soldering the flange to the vacuum system. A second way of joining components is the use of compression fittings that are available from vacuum-equipment manufacturers. The third (but least satisfactory) way is to join the components with a short length of vacuum hose, using vacuum grease and radiator hose clamps.

"The second problem that is likely to trouble the amateur is the selection of valves. In general we try to avoid the use of valves altogether. If they are necessary, then several solutions are possible. For small applications (e.g., the admission of gases) standard Hoke valves will usually do the job. Larger valves are best purchased from vacuum-equipment manufacturers; however, conversions can be made to other types. A Hills McCanna valve, for example, can be modified by first removing all paint inside, polishing the inside sealing surface with fine emery cloth and finally thoroughly cleaning all inner surfaces.

"Consideration should also be given to the system design. All vacuum lines should be kept as short arid as large in diameter as possible, with a minimum of valves, obstructions and unsoldered joints. A 1-inch or 1 1/2-inch inside-diameter tube will usually provide adequate conductances for the high-vacuum manifold, while 1/2-inch to 3/4-inch inside-diameter tubing is usually sufficient for roughing lines. If oil or mercury systems are used, a cold trap may be desirable, and this can be obtained by refrigerating part of the high-vacuum line. Small electrical connections can be made by sawing off the bulb of an old metal radio tube and soldering this into the vacuum system. The tube connection pins can be used to make electrical connections to parts inside the vacuum.

"When the construction is completed, the parts should be thoroughly cleaned, with copper parts bright-dipped and thoroughly rinsed, then dipped in or rinsed with alcohol (or acetone) and dried. An effective dipping solution can be made by combining the following ingredients in the order listed (by volume): distilled water 18 per cent, sulfuric acid 61 per cent and nitric acid per cent. E a bright finish is desired, add 10 drops of hydrochloric acid per quart of solution.

"The parts should then be assembled and the vacuum system started. Gentle heating oú the tubing with a torch or even a hair drier will help in removing water vapor and other gases from the inner surfaces of the system and thus will aid greatly in improving the vacuum that will eventually be obtained. Patience is essential. Outgassing of a new system may take a few hours or several days. Experience has shown that it does no good to rush the process. Don't panic if the vacuum is poor when the system is first started. Things will gradually improve. Such a system should easily reach a pressure of 10^-6 millimeter of mercury. We usually design equipment (using this type of construction) for pressures of 1 X 10^-7 millimeter if valves are used and 2 X 10^-8 millimeter if no valves are used.

"A final word of caution concerning the selection of materials to be used in the vacuum: Glass, ceramics and metals with low vapor pressure such as copper, tungsten, tantalum, titanium, stainless steel, tin, Nichrome, gold and silver will give no trouble. High vapor-pressure metals such as zinc, mercury and brass should be avoided. Organic substances, oils and plastics generally are totally unsuitable; however, if plastics must be used, then Teflon is as good a material as any. If in doubt, consult vapor-pressure data before using a material. The use of some high-vapor-pressure substances may contaminate the system and necessitate its complete disassembly and cleaning."

The ordinary microscope is obviously a one-man instrument. Viewers must take turns at the eyepiece, a handicap that seriously limits the collaborative study of specimens. With the addition of a few simple accessories, however, even inexpensive instruments can be adapted for microprojection and photomicrography. Both living protozoa and permanent slides can be enlarged to the proportions of a home-movie screen. Similar adaptations enable you to make large-scale photographs of slides, either in black and white or color. Gene Udell, associate professor of education at Temple University in Philadelphia, explains how to proceed:

"Microprojection," he writes, "consists essentially in 'backfiring' light through the microscope. To be effective, the image must emerge through the eyepiece with sufficient intensity to be visible as a projected image upon a screen.

"A 85-millimeter slide projector affords a good source of illumination for the experimenter. So also does an 8-mm. Or 16-mm. motion picture projector. The projector is angled down to send its light beam as directly as possible onto the reflecting mirror of the microscope. A slip of ordinary writing paper is placed upon the microscope stage; the mirror, light source, diaphragm and substage condenser, if any, are adjusted until the paper over the stage aperture shows the most intense light. The paper is then replaced with a slide and the light beam proceeds through the optical system of the microscope. Careful focusing by bringing the microscope objective close to the slide and then backing it off will provide a clear image on the ceiling.

"The image can be diverted at a right angle for screen viewing by resting a 90degree prism directly upon the microscope ocular. Alternatively a small first-surface mirror, such as the No. 40,242 that is distributed by the Edmund Scientific Company of Barrington, N.J., can be substituted for the prism. It makes a brighter image than a prism and costs about 50 cents. A bracket to support the mirror can be made of any convenient sheet metal.

"Stray light from the projection lens and the air-circulation vents of the projector can be masked from the screen by putting the complete setup in a cardboard box. A hole is made in the top of the box for the lens tube of the microscope and a door in one side provides access for changing slides and focusing. For extended projection the cardboard box can be vented and baffled to provide air circulation.

"Several refinements can be added as desired. A simple condenser lens, installed between the light source and the reflecting mirror of the microscope, will direct additional light into the optical system and increase the screen illumination. A cell of copper-sulfate solution in the path of the light source will absorb much of the heat and so prolong the active period of living cultures. The copper-sulfate solution will not noticeably reduce the illumination.

"Up to this point the assumption has been that the projection will be directed onto a standard reflective screen surface. When this is the case in a room capable of being completely darkened (using a light source of 800 watts to 500 watts), a clear image at least four feet in diameter can be achieved. Should stray light be a problem, an excellent image can still be obtained by utilizing rear-screen projection. Cardboard sections cut in the form of a square-sided trumpet or megaphone, with a piece of sturdy milky plastic or plain window glass backed with waxed paper set in the large opening, will provide unusually effective viewing. Properly supported, the screen can be swung in an arc to include all viewers. This setup can also be used as a tracing surface for tracing outlines of still subjects. Tracing paper placed against the glass or plastic screen will show a clear picture of the projected slide. The light emerges from behind the pencil or pen without complicating shadows.

"Living specimens are dramatically conveyed to the screen. A hydra will seek out a water flea, grasp it with a tentacle, paralyze it and begin the ingestion process. Paramecia in a drop of rich culture will swarm over the screen. Strands of absorbent cotton will impede motion to permit examination of single specimens. Among the algae, spirogyra shows its delicate internal structure clearly. The experimenter is directed to standard texts for methods of establishing cultures of microorganisms.

"A word should be added concerning the effects of magnification upon projection. As is evident, best projection, in terms of illumination, is obtained at low magnification. Oculars of three-diameter and five-diameter magnification are available at moderate cost should the existing ocular of the microscope provide too much magnification for successful illumination. At close range even oil-immersion magnification will be seen brightly with the arrangements described.

"Now to photomicrography. This means of recording the contents of slides on film and paper utilizes the same basic arrangement of backfired light.

"In the simplest case the projected image is cast upon a piece of photographic projection paper in a totally dark room. Since the microscope and light source are enclosed, there should be no difficulty. Limited amounts of stray light can be tolerated in the room if slow emulsions are used, such as projection paper emulsions.

"Often a paper negative will be fully as clear as a black-and-white standard print. Sometimes it is even more useful. If a positive print is desired, the paper-negative image can be placed in contact with the emulsion side of another piece of paper and exposure made through the paper negative. This yields a positive paper print. Incidentally, I cut my costs during my early experiments by buying slightly overage paper.

"To make film negatives and photographic enlargements, the light source and microscope are set up in the same way. A folding camera with lens removed and back opened is set on time exposure and placed over the microscope ocular so that the light path continues on through the camera as shown in the accompanying illustration [Figure 3]. Appropriate supports can be improvised for the camera. Across the open back of the camera two sheets of glass are laid, sandwiching a piece of waxed paper. The image is focused on the translucent paper. The light is then turned off, and a piece of cut film is substituted for the waxed paper. The light source is flicked on and off; the film is removed and developed. Exposure times are a matter of test, light intensity being regulated by the camera diaphragm.

"A different way to approach film exposure is by using a dummy camera. The dummy camera is a box with a piece of etched glass serving as its back. Since the eyepiece of the microscope serves as a lens, no lens is needed for either the regular camera or the dummy viewing camera. So long as both are adjusted to give a sharp image under the same conditions, focusing can be done with the dummy camera, which is then replaced by the real camera. The focusing of the latter is done by substituting waxed paper or etched glass for the film. Once the focuses of the two cameras are matched, the real camera can be loaded. This eliminates the need for a dark room.

"If the back of the real camera opens fully or can be removed, the instrument can be made to do double duty as an enlarger. Cut a hole in the bottom of a cigar box to match the open back of the camera and tack two wooden strips (shelf supports) inside the box about an inch above the bottom. Two clear glass panes, hinged with tape along one edge, rest on the supports. A circular hole is cut in the lid and is fitted with a tin can equipped with a socket and a photoflood bulb (No. 211 or No. 212). The top of the can must of course be closed to prevent stray light from shining into the room. It may be necessary to fit the lower hole with a frame (to serve as a light trap that fits into the back of the camera). To use this arrangement as an enlarger make a vertically adjustable support for the assembly that can center the lens over a piece of white cardboard. Insert a negative between the panes of glass, turn on the light and operate the focusing adjustment of the camera so that a sharp image of the negative is projected onto the cardboard. Then turn off the light, replace the cardboard with a piece of projection paper and make an exposure. Exposure time must be determined experimentally. The accompanying photographs show the quality of the results that can be achieved."

Bibliography

GEOMETRICAL OPTICS. L. C. Martin. Philosophical Library, 1956.

GUIDE TO THE MICROSCOPE. Arthur Beiser. E. P. Dutton & Co., Inc., 1957.

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