THE ART OF STAGE LIGHTING HAS come a long way since the days of the showman P. T. Barnum and the microscopist Ernst Abbe. These pioneers knew that organisms, whether human or microscopic, rarely look their best in the head-on glare of white light. Both workers recognized the advantages of proper background illumination and learned to bring out modeling through the use of oblique light. The intervening decades have brought their techniques to a high state of development. Today's impresario can multiply his effectiveness by-mastering these techniques whether he presides over the stage of a theater or that of a microscope.

"When I started to use my first microscope," writes John De Haas, an amateur microscopist of New York City, "I thought the only way you could see significant detail in an animal of microscopic proportions was to kill and stain it. I was interested in studying protozoa, particularly the flagellates. But the application of make-up to my performers ended the show before the curtain went up. I wanted to study the internal structures of my animals as they swam, fed and reproduced. That meant that I had to work with light alone. As things developed, Rheinberg color illumination, dark-field lighting and even an inexpensive version of phase-contrast microscopy all proved within easy reach of my facilities. Astonishing increases in effective resolving power, I learned, are possible even with bright-field illumination. You simply block off 95 per cent of the light from certain directions. Beginners often make the mistake of flooding their specimens with light and thereby washing out all detail.

"The effect is easy to demonstrate. Assume that you have a medium-priced microscope fitted with an eight-millimeter objective, an Abbe condenser and an eyepiece capable of giving the instrument a magnification of 150 diameters or so. Put a slide of diatoms under the objective. Focus on one of them carefully and fiddle with the lamp, mirror and condenser adjustments until maximum detail appears. The chances are good that you will see the diatom in sharp outline against the bright field. The body will show little detail beyond a fuzzy pattern of striations. Substituting an eyepiece of higher power will not help much. The striations will appear bigger-but proportionately fuzzier. At this point many amateurs decide they need a better microscope. There is another way out, and it costs much less.

"Cut a disk of black cardboard to fit the filter carrier of the substage and make a quarter-inch hole in the disk midway between the center and the edge. Without disturbing the adjustment of the instrument, slip the disk into the substage. You will find a position where, despite the lower intensity, an astonishing amount of detail comes into view. The striations stand out sharp and clear and, depending upon the structure of the diatom, you will observe that the striations are rows of objects still more minute. By moving the disk slightly up and down or sideways, or rotating the stage, even smaller details can be resolved."

This technique, De Haas explained, is a form of oblique bright-field illumination. Nothing is coming though the eyepiece that was not there before. When the iris of the condenser is wide open, contrasting details of light and shadow in the image are submerged in glare because the deviated rays comprising the first- and lower-order spectra, that carry the fine details of the specimen, are faint when they are compared with the direct or zero-order rays.

It is interesting to examine and manipulate the spectra because this enables you to predict the resolution even before you look at a specimen. To view the spectra you observe the back lens by removing the eyepiece and substituting a pinhole. You will also need a series of opaque disks punched with a quarter-inch hole, the position of which progresses through the series from the center to the edge.

With the iris of the condenser wide open, focus the microscope on the diatom as described in the De Haas experiment and look through the pinhole at the back lens of the objective. The lens will be flooded with light to its edge. Now slowly close the iris. As the image of the iris in the back lens becomes smaller and smaller, the edges of two colored disks will appear at opposite sides of the field. The edges nearest the white disk will be an intense violet that shades into blue. These are images of first-order spectra. The fine structure of the diatom is acting as an amplitude diffraction grating, and hence some of the rays are deviated from the central beam. Now insert one of the opaque disks which has been perforated slightly off center into the filter holder and open the iris wide. The radius on which the perforation lies should be lined up with that of the spectra. The pinhole will now show that one of the colored images has in effect advanced into the field of view, while the white image and the other colored one have retreated an equal amount. Substitute in the filter holder the remaining opaque disks of the series one at a time for the initial disk. In effect this moves the perforation successively farther from the center. Observe that as the first-order spectral image advances more and more into the field, it shades through the colors of the rainbow and that, as it comes fully into view, a fainter companion appears at the edge. This companion is the second-order spectra. Depending upon the characteristics of your objective and the specimen, you may be able to entice as many as three orders into view before the white zero-order disappears. Moreover, by rotating the specimen or adjusting its lateral position slightly you may be able to bring other spectra into view at right angles to those already in the field. When you have accomplished this, substitute an eyepiece for the pinhole. The specimen will bristle with detail!

The explanation of why high resolution is associated with spectra is found in the wave nature of light. Except for shadows of gross objects cast by zero-order rays, the microscopic image is formed by interference at the focal plane of the eyepiece between rays that have been diffracted at the object, as shown in Roger Hayward's drawing at the upper right on this page.

The technique of increasing effective resolution by the use of an off-center diaphragm may be considered a form of annular illumination, in which the specimen is lighted by a hollow cone of light. Most of the cone is missing in the above experiment, but it would be complete if a ring of perforations were extended all around the opaque disk. A luminous cone would then surround a solid cone of darkness. If the base of the dark cone were made wide enough, no direct light from the condenser would enter the objective; and in the absence of a specimen, the field would appear dark. A specimen would reflect some of the rays into the objective and thus cause it to stand out in bright contrast against the dark field. Such reflection is greatest at points where the refractive property of the specimen changes abruptly, as at its edges. Hence the usefulness of dark-field illumination is limited to specimens characterized by sharp contrasts in refractivity.

This is also true, for the same reason, of Rheinberg color illumination. Although Rheinberg illumination has limited value as a research tool, the fascinating results justify setting it up as an experiment. Essentially it is annular illumination in which an outer cone of colored light surrounds an inner cone of light in a contrasting color. As in the case of dark-field illumination, unless a special condenser is used, Rheinberg illumination works best with low-power objectives, preferably those of 16 millimeters or more used in combination with a five-power eyepiece. To prepare a Rheinberg setup, cut a disk of colored gelatin or other plastic to fit the filter holder. Red is a good color for this disk. Perforate the center with a half-inch hole. Next cut a five-eighths-inch disk of contrasting color, say blue. Cement this disk over the perforation in the larger disk. The difference in size between the small disk and perforation allows for a sixteenth-inch overlap. Insert the assembly in the filter holder and rack up the condenser until it is focused on the object plane. The field will now appear uniformly blue because the objective is immersed in the central blue cone of light. Now place a small amount of water on a slide and drop in a grain of some effervescent substance such as Bromo Seltzer. The eyepiece will present a striking display as myriad bubbles bend red rays into the objective-like fireballs rising through a sea of blue.

Of greater interest to the advanced worker is the relatively new technique of phase-contrast microscopy, devised in 1934 by the Dutch physicist Frits Zernike. Phase contrast, like dark-field lighting, may be considered a form of annular illumination. Unlike other forms of annular illumination, however, Zernike's technique produces contrasts in the image by exploiting minute differences in phase between direct waves from the light source and those deviated by the specimen. On the opposite page Hayward depicts concentric waves of light radiating from what may be considered a point source. The amplitude or brightness varies inversely with the square of the distance from the source, as suggested by the sine wave extending to the left from the lamp. The velocity . of waves intercepted by the glass block (upper left) is retarded by the refractive property of the glass and, in this case, the light emerges from the block a full wavelength behind that propagated through the surrounding air. Although both sets of waves are spherical, those that have traversed the glass block are now said to "lag" 360 degrees behind those of the source. The lens (upper right) intercepts the spherical waves and, by virtue of its thickness increasing toward the center, retards the spherical wave fronts just enough so that they emerge as plane waves.

Many microorganisms are transparent and, like glass, can retard the velocity of light. Unfortunately the refractivity of many interesting ones nearly matches that of the medium in which they live. The eye is sensitive only to changes in amplitude or brightness. Hence microorganisms that cause only small differences in phase between light transmitted through them and through the surrounding material are invisible. Prior to the invention of phase microscopy they could be seen only after being stained or immersed in a fluid of substantially differing refractive index. These alternatives either killed them or seriously interfered with their natural processes.

The phase-contrast technique makes such objects visible by transforming small differences in phase into small differences in amplitude or brightness-to which the eye is sensitive. The trick is accomplished by retarding part of the light (passing it through a thin sheet of glass or similar material called a phase plate) so that all light arrives at the focal plane of the eyepiece in phase or 180 degrees out of phase. The crests and troughs of the waves are thus made to coincide or cancel. They "interfere," or combine their energies, and thus set up amplitude differences that constitute an image of the object.

The fine details of the image are carried by spectral orders of phase. These correspond to the spectral orders of amplitude that account for image resolution in ordinary bright-field work, as demonstrated by the experiment in oblique lighting and illustrated by Hayward's drawing of an amplitude grating on this page. A phase grating, consisting of alternate strips of transparent material which differs slightly in refractivity from the intervening material, works much like the amplitude grating illustrated in the top drawing on this page. Transparent specimens can be considered phase gratings because the refractivity of their structure varies; rays transmitted through those portions of higher refractivity are deviated with respect to those transmitted by portions of lower refractivity. Thus two sets of waves enter the objective, distinguished only by their phase difference. It can be demonstrated mathematically that a third wave can be found which represents the phase difference between the direct and deviated ray. This difference wave is always just 90 degrees out of phase with the wave emerging from the specimen. A value of amplitude can be assigned to the difference wave so that when it is added to the wave deviated by the specimen, the sum equals the direct wave transmitted by the surrounding material. Zernike looked for the difference wave in nature -and found it! It is the phase spectra set up by the specimen and it carries the specimen's phase image.

As observed in the experiment with oblique lighting, the spectral orders spread across the aperture of the objective's back lens and converge at the plane of the eyepiece. The condenser is adjusted so that direct rays from the source meet at the back focal plane of the objective, as shown by the top drawing on this page. Hence rays which pass through the center of the back focal plane and those transmitted through the complementary area ( the spectral orders) are out of phase by 90 degrees. When this difference is adjusted so that the two arrive at the focal plane of the eyepiece precisely in or out of phase, interference takes place and an amplitude-contrast image results as in conventional bright-field illumination.

Zernike corrected the phase difference by inserting an annular phase plate in the path of the direct wave, as shown in the bottom drawing on this page. In addition he coated the plate with a layer of light-absorbing material just dense enough to reduce the intensity of the direct wave to that of the deviated wave so that complete addition or cancellation would take place. This prevents the image from being masked by the excess light of the direct wave. As subsequently improved, plates are designed either to advance or in effect retard the direct wave and thus result in either constructive or destructive interference. Constructive interference causes a bright object to appear against a neutral background. Destructive interference reverses the effect. A set of phase plates, together with accessories including a condenser annulus and a telescope for aligning elements in the optical train sells for about $200. De Haas devised a form of phase contrast for the amateur which approximates the results of the Zernike technique and costs less than $20 It works well where phase difference amounts to a 20th of a wave or more.

De Haas's project was supported by a grant from the Pennsylvania Academy of Science and was reported in the Academy's journal last year, His setup requires a three-element Abbe condenser with the top lens removed, an iris or Davis diaphragm for use with a four-millimeter objective of the dry type, a 16-millimeter objective and individual stops (made with opaque lacquer on daylight filters of medium shade) for each objective. These are inserted in the filter carrier beneath the substage condenser. The size of the stops must in general be determined experimentally. For a 16millimeter objective of .25 numerical aperture the stop diameter should be 19/64 inch. A four-millimeter objective of numerical aperture .65 requires a stop of 21/82 inch.

An appropriate stop is inserted in the filter holder, the eyepiece removed and the image of the stop carefully centered by eye in the back lens of the objective. The lamp must also be centered on the optical axis. A specimen is then placed on the stage and focused. A position of the condenser will then be found where details of the specimen stand out in sharp contrast-lighter or darker than the surrounding field depending upon the adjustment. When using the four-millimeter objective, the position of the Davis iris influences the result and the best adjustment must be found by trial and error.

The technique contrasts with dark-field microscopy, where the condenser must lie in exact focus at the object plane and in which the object is always brighter than the field. The De Haas system appears to work on the principle of fringes diffracted by an edge-the stop providing the edge. The objective diaphragm probably acts on these fringes and those from the specimen so that they get added as in the Zernike system. De Haas has not attempted a theoretical analysis of the system. His optical train is shown Figure 6. Photomicrographs showing a specimen of Amoeba limax made with ordinary transmitted light and by the De Haas system appear in Figure 5.

In general the problem of achieving the desired stability has been attacked in two ways. The first reduces the difficulty by exchanging a pronounced slow wobble for a less-pronounced fast one.

Low amplitude is bought at the price of high frequency. Axes are made heavier and overhang is reduced to a minimum. In other words the "pendulum" is stiffened and shortened. It is possible to increase the rate of vibration of six-inch instruments to as much as 10 cycles per second by using stubby shafts for axes and appropriately light materials in the construction of the tube and its accessories. However, even this relatively high frequency is well within the range of a good photoelectric recorder. It can easily mask the subtle features of occultation recordings. Thus most designers tackle the problem by the second method: increasing the effective diameter of the axes. A now-classic example of this type of design is represented by the Springfield mounting introduced 85 years ago by the late Russell W. Porter and described in Amateur Telescope Making-Advanced. But even though the Springfield axes are defined by plates as broad as the telescope's mirror, physicists of the Army Map Service found it necessary to substitute oversized ribbed castings before the design would yield useful occultation records. Perhaps the simplest way of increasing the effective diameter of axes is to adopt one of the numerous yoke designs. They are cumbersome, of course, and troublesome to move. But if you are willing to sacrifice portability for stability of operation, a good yoke mounting is your dish.

W. P. Overbeck, of Aiken, S. C., has built one that he swears can function simultaneously as a trapeze and a precision photometer. He built the mounting for far less than the patterns alone of a Springfield would cost.

"My enjoyment of astronomy," he writes, "lies in calculating positions of objects or timing astronomical events and subjecting the results to accurate measurement. This provides relaxation from other activities which are not capable of such precise evaluation. Of course the professional astronomer does not define astronomy in such simple terms; he is most interested in phenomena which are very difficult to measure. But the amateur can take delight in finding things within a small fraction of a degree of where he expects to find them or in timing events within a few seconds, particularly when he has built his own instrument.

"To satisfy the desire for accuracy, I soon came to the conclusion that it would be necessary to build a telescope which could be permanently, and very solidly, mounted outdoors. It was difficult to see how reproducible measurements could be obtained with a portable telescope. Secondly, having meager facilities for precision machining, it became clear that I must use a type of mounting in which the principal axes are supported on widely spaced bearings so that any play in the bearings would have a minimum effect on angular motion. Finally, a permanent outdoor mounting must be weatherproof unless one wishes to add the cost of building an observatory complete with dome."

The answer-to all these requirements is shown Figure 7. Writes Overbeck: "The telescope tube and its declination axis are mounted in a yoke with bearing points spaced about a foot apart. The yoke is similarly mounted between two heavy A-frames with bearings about eight feet apart. With a 'slop' of less than .0005 inch in the bearings, the declination axis is fixed to within .0024 degrees and the polar axis to within .0003 degrees. The A-frames are fastened down with heavy bolts set in a four-inch concrete slab. The arrangement is so solid that one can climb on the frame while making observations. The principal timbers of the mounting are four by six inches. They were measured and cut with considerable care to obtain the correct angle of tilt for the latitude of the site. Each piece was then thoroughly shellacked and painted and finally assembled with half-inch lag screws, most of which were six to eight inches long.

Preliminary alignment of the slab and frames was done by first establishing a true north-south line. This was accomplished by suspending a long plumb line from a temporary support and marking off the position of its shadow as the sun passed through the meridian. By referring other measurements to this line, it was possible to get everything lined up to within a small fraction of an inch and minimize the more precise alignment work required later.

"The yoke was built of half-inch plywood, heavily reinforced with two-by-six and two-by-four-inch pieces at the points requiring strength. The sides of the yoke, as well as the main A-frames, are built so that they provide 'bearing boxes' at the points where the bearings are to be located."

The detail at the lower left of Figure 7 shows the arrangement of bearing, clutch and driving gear at one side of the telescope tube. The bearing is made up of three-quarter-inch pipe fittings which were machined down to get a good fit and alignment. Starting from the right, the detail shows a partial section of the tube, which at this point has a double wall for strength. A flange is fastened to the tube and holds a tubular bearing shaft made from a six-inch by three-quarter-inch pipe nipple. The shaft passes through-a second flange which is fastened to a two-by-four-by-eight-inch wood block which fits snugly inside the bearing box and is held in position by two lag screws. The lag screws pass through clearance holes in the sides of the box, permitting a small amount of adjustment. On the outer end of the bearing shaft is a third flange, faced with rubber, which forms half of a disk-type clutch. The other half of the clutch is separately assembled with the driving gear on a steel bushing which slides into the outer end of the tubular bearing shaft.

Overbeck continues: "To control the clutch, a long quarter-inch steel rod, having a collar fastened to it, extends through the center of the entire assembly and is threaded into a steel plug at the inner end of the bearing shaft. In the actual assembly the two clutch faces remain in contact. They are surfaced with thin, smooth gasket rubber and require very little release of pressure to change from a locked condition to one which permits free motion. The drive for both declination and right ascension is provided by miniature electric motors which are geared 500,000 to 1 and are housed in the bearing box along with the clutch and gear.

"This makes it easy to maintain good alignment between driving gear and shaft. The box is sealed by a rubber strip which is tacked inside a four-inch hole in the face toward the telescope and which fits snugly against the side of the telescope. A second seal closes the opening around the clutch control rod. It took much longer to invent this assembly than it did to build it. One of its greatest advantages is that it can easily be taken apart, piece by piece, from the outer face of the box.

"The assembly above described is duplicated at one end of the polar axis. The other two bearing assemblies have no clutch or gear but simply have a graduated circle fastened to the flange which forms the inner clutch-face. The outer covers for these bearing assemblies have glass inserts."

The telescope tube is made of half-inch plywood reinforced by several internal "ribs" which are pieces of plywood seven inches square with six-inch holes cut in them. When the tube is not in use, both ends are covered with simple plywood lids such as the one shown in the detail at upper right in the drawing.

A plywood box for carrying the eyepieces is also shown in the drawing. It was carefully built to the same dimensions as the tube. The mirror assembly can be used as a lid for the box when storing the optical parts indoors, or may be quickly installed in the end of the telescope when one wishes to make observations. The diagonal mirror and spider assembly at the other end of the tube is well protected from the weather and remains installed at all times.

Says Overbeck: "After final assembly, precise alignment of the telescope becomes a simple matter. First, the optical axis of the mirror was aligned with the dimensional axis of the tube. This was done by careful sandpapering of the end of the tube that supports the mirror. The alignment is checked by looking through the tube at the mirror from a distance about equal to twice its focal length. From this point one can see an enlarged image of the pupil of one's own eye which, when the mirror is aligned, will be neatly centered on the mounting spider of the diagonal mirror. (My mirror has a small hole in it, when the diagonal is removed, which makes this test very precise.) Second, the polar axis may be aligned by following a star, preferably one near the celestial equator, across the sky and observing apparent changes in declination. This was repeated several times until no apparent change could be seen. Finally the declination axis was checked by making several observations of stars of varying declination to find if there were consistent errors in relative values of right ascension.

"The first adjustments were made by calculating the required motion of various supports and bearings and later adjustments were made 'by feel' during observation. The procedure was successful in bringing the alignment of both axes within about .08 degree of perfect positioning as determined by averaging many individual measurements. This is much better than the precision with which it is possible to read the declination and hour circles, so for practical purposes it is more than adequate.

"Thus the final result is an instrument which is capable of more precise measurement than most telescopes in the $500 to $1,000 price range, which does not suffer from vapor condensation and: thermal convection currents as most metal telescopes do and for which the total cost of materials was less than $100. It should be realized, however, that it cost a great deal of time and effort because its precision is largely due to its heavy structure and to the careful attention given to measuring, cutting and finishing of each piece of wood to insure both accuracy and long-term stability of dimensions. For me the effort was well worthwhile in meeting the particular objective I had in mind."

t is a good bet that no amateur telescope maker gets halfway through his first telescope mirror without wishing that he lived in Springfield, Vt. Three miles outside Springfield is a fir-clad knob called Breezy Hill, the site of the little clubhouse-observatory called Stellafane. This pleasant spot is periodically the epicenter of amateur astronomy. At a chosen time each year amateurs from everywhere get together to "talk and talk and look and eat." These words are quoted from Jim Gagan, chairman of the Stellafane Committee of the Amateur Telescope Makers of Boston. It is this group which arranges the meeting.

Writes Gagan: "The 1955 meeting will take place on August 20. The Vermont corn will be ripe, the moon will be three days old and good seeing (9G per cent) is predicted.

"Come yourself and pass the world along. If you like to rough it, you can camp on Breezy Hill. If not, room reservations can be made through the Hartness House, Springfield, Vt."

Bibliography

PHASE MICROSCOPY. Alva H. Bennett and other. John Wiley & Sons, Inc., 1951.

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

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

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/