ATOMS AND MOLECULES, WHEN struck a sharp blow by a hammer of atomic dimensions, ring like bells. The ear is not sensitive to the electromagnetic waves they emit, but the eye is. All light originates in this way. Just as every bell makes a characteristic sound of its own, depending upon its size and shape, so each of the hundred-odd kinds of atoms and their myriad molecular combinations radiate (or absorb) light of distinctive colors. The instrument physicists use to sort out the colors, and thus identify substances, is the spectroscope. This powerful instrument is relatively simple in principle and not difficult for amateurs to build. Yet it is one of the most useful tools in the scientist's kit. During the first half of this century it has helped to answer an incredible number of scientific questions-more than the telescope and microscope combined.

Accordingly, spectroscopy has become a specialized and important branch of physics. Any substance, whether a piece of cheese or a rusty hairpin, can be made to emit light by heating it. When the resulting light is sent through a spectroscope, the rays separate into lines or bands of colors which not only tag the responsible atoms but may reveal many secrets of how they are combined.

Light waves are only about one 50,000th of an inch long. With a good spectroscope, however, you can measure their size within a few trillionths of an inch, or about a billionth of the thickness of the paper on which this magazine is printed. With this information as a guide, chemists have learned how to take molecules apart and reassemble them into substances with new and desirable properties. About 20 kinds of molecules have been manufactured in the laboratory for every one chemists have identified in nature. The tool contributing most to this analysis and synthesis is the spectroscope.

Isaac Newton laid the foundations of spectroscopy when he observed that a prism bends rays in the blue end of the spectrum more than those nearer the red end. On February 6, 1670, Newton wrote Henry Oldenburg, then secretary of the Royal Society:

"To perform my late promise to you, I shall without further ceremony acquaint you that in the year 1666, I procured me a triangular prism, to try therewith the celebrated phenomena of colors. And in order thereto, having darkened my chamber and made a small hole in my window-shuts, to let in a convenient quantity of the Sun's light, I placed my prism [so that the ray] might be refracted to the opposite wall. It was at first a very pleasant divertissement to view the vivid and intense colors produced thereby, but after awhile, applying myself to consider them more circumspectly, I became surprised to see them in oblong form which according to the laws of refraction, I expected should have been circular.... I took two boards and placed one of them behind the prism at the window so that the light might pass through a small hole made in it for the purpose and fall on the other board which I placed about 12 feet distance. Then I placed another prism behind this second board so that the light trajected through both boards might pass through that also.... This done, I took the first prism in my hand and turned it to and fro slowly about its axis so much as to make the several parts of the image cast on the second board successively pass through the hole in it that I might observe to what places on the wall the second prism would refract them. I saw . . . that the light on the (violet) end did in the second prism suffer a refraction considerably greater than the light tending to the other end (red) . . . and that according to their particular degrees of refrangibility they were transmitted through the prism to divers parts of the opposite wall."

With this demonstration Newton's service to spectroscopy came to an end. He apparently failed to see any of the fine detail which gives the spectrum its significance. Nevertheless he subsequently stated that the sorting of the colors could be carried further by the use of improved prisms and lenses.

It is difficult to understand why Newton did not reduce his spectroscope to convenient laboratory form. He almost had it in principle.

Yet he and his successors were content to work in darkened rooms for nearly two centuries. Even Joseph von Fraunhofer's epoch-making discovery of the dark absorption lines, which split the solar spectrum into thousands of parts, was made with a setup that filled his laboratory. To view the slit in the "window-shut," beyond his prism, Fraunhofer used a theodolite telescope placed behind the prism-an improvement which enabled him to make good measurements of the angles through which the light was bent. The clear view of the slit thus afforded disclosed "an almost countless number of "strong and weak vertical lines," which close examination proved were "in the sunlight." Fraunhofer could not explain the lines, but he made an accurate chart of about 700 of them and designated eight of the most prominent ones by the letters A to H, by which they are still known.

The meaning of the dark lines remained a mystery until 1859, when they were explained by the Heidelberg physicists Robert Bunsen and Gustav Kirchhoff. They made the profound discovery that gases through which a ray of light ' passed would absorb certain narrow portions or colors of the light. The absorptions were signaled by dark lines in the spectrum. They also demonstrated that if the absorbing gas itself was heated to incandescence, then the dark lines of absorption became bright lines of emission, which stood out on the dark spectral band if there were no other source of light.

In the course of their experiments Bunsen and Kirchhoff reduced the size and design of the prism-type spectroscope to substantially its present form. Amateurs who would like to experiment with one may enjoy building the instrument according to the directions written by Bunsen 96 years ago.

"It is well known," he wrote, "that many substances have the property when they are brought into a flame, of producing in its spectrum certain bright lines. We can base a method of qualitative analysis on these lines that greatly broadens the field of chemical research and leads to the solution of problems previously beyond our grasp.

"The gas lamp previously described [Bunsen's gas burner] gives a flame of high temperature but low luminosity. Into this flame we introduced for investigation a small quantity of chlorate of potassium which had been recrystallized six or eight times. The apparatus we have used for investigating spectra is shown [see illustration on the opposite page]. The box [holding the prism] is blackened on the inside. Its two inclined sides carry two small telescopes. The ocular of the one facing the test flame is replaced by a plate in which is a slit formed by two brass blades. The burner is placed before the slit. The end of a fine platinum wire, bent into a small loop and supported by an apparatus stand, passes into the flame; on this hook is melted a globule of the chloride previously dried. Between the objectives of the two telescopes is placed a hollow prism with a refracting angle of 60 degrees and filled with carbon disulfide. The prism rests on a brass plate that can be rotated by a vertical shaft. The shaft carries on its lower end a mirror, above which an arm attaches which serves as a handle for turning the prism and mirror. Facing the mirror is another small telescope arranged to give an image of the horizontal scale, placed a short distance away. By rotating the prism, one can make the entire spectrum of the flame pass before the vertical cross-hair in the ocular of the viewing telescope. To every point in the spectrum, there corresponds a certain reading of the scale."

With this instrument Kirchhoff and Bunsen made the series of elegant investigations which founded the modern science of spectroscopy. Their explanation of the Fraunhofer lines and discovery of the elements cesium and rubidium inspired scientists all over the world to take up the new field of investigation and raised public interest in science to a high pitch.

Unfortunately Bunsen omitted one critical detail of construction that has plagued instrument makers ever since. He failed to specify the kind of cement he used to join the glass slabs of his hollow prism and seal in the foul-smelling, volatile, explosive and 1:oisonous carbon disulfide.

R. B. Nevin of Christchurch, New Zealand, has made Bunsen-type prisms with wax as the sealer. He describes them as follows:

"My 60-degree prisms are made of eighth-inch plate glass which has never heard of such a thing as a figure. The glass is cut into pieces 2.5 by 2 inches, accurately oblong. Using a carborundum stone, I bevel the long edges slightly on one side at an angle of 30 degrees to the horizontal, and similarly bevel both sides of the shorter edges. The three slabs are then assembled on the bench as an equilateral triangle, the bottom one being stuck with anything handy if it won't stay put. A few bits of sealing wax are put in the top groove, and a gas flame is gently wafted along the glass until the wax melts. It is then spread with a match stick and given more heat until liquid. Next it is coaxed firmly but gently into the groove with the stick. More wax is applied until the groove is filled level with the glass edges. Then the whole is allowed to cool. A Bunsen burner flame about half on and just nonluminous is correct. If the flame is too intense, it heats the glass unevenly and cracks it The procedure is repeated for the remaining two grooves. If the glass has been cut squarely and beveled properly, the edges will fit without adjustment. The wax holds to the ground edges tenaciously and is easy to flake off the polished surfaces where it is not wanted.

"The assembly is very strong for its dimensions. A triangular bottom for the box is then cut about a sixteenth of an inch larger than the assembled walls. The upper edges, upon which the walls will rest, are similarly ground, beveled and cemented to the previously completed subassembly. Gentle finishing touches with the flame to give a smoothly finished job can be applied to taste. The important precaution is: Do not rush the job. The sudden application of heat to one spot will crack the glass.

"When the prism is thoroughly cool, water can be poured into it, and when you look at a source of white light through it you will see all the colors of the rainbow. Glycerin in place of water will improve the prism's definition slightly, although its dispersion is about that of water and of crown glasses. It is significantly lower than flint glass. Perhaps the best liquid of all is carbon disulfide. Unfortunately it is a splendid solvent for sealing wax!

"That's about all there is to it. I whipped through a quite serviceable prism the other day in about 30 minutes of careful work-from cutting the glass to filling it with water."

Liquid prisms offer a number of distinct advantages to the amateur. They are easy and cheap to make, especially in large sizes. They also allow a wide choice of materials and dispersions. Their principal disadvantage is that the dispersive power of liquids varies greatly with changes in temperature. Moreover, temperature gradients within the liquid create inhomogeneities in dispersion with a consequent loss of resolution. The Fraunhofer lines blur and merge unless the liquid is maintained at uniform temperature.

Roger Hayward, after making the drawing of Bunsen's spectroscope shown here, volunteered a few practical tips out of his experience with liquid prisms. "Carbon disulfide," he wrote, "is terrible stuff. In addition to being smelly and explosive, it is a particularly insidious, chronic poison which can produce permanent damage to the spinal cord and brain. It is like playing with a bunch of uncaged cobras. Never handle it except under a ventilating hood.

"The best substitutes for carbon disulfide from an optical point of view are monobromonaphthalene, ethyl cinnamate (expensive), an aqueous solution of barium-mercuric bromide and oil of bitter almonds, in that order. The prism should be made with a glass bottom. The difference between the temperature coefficient of glass and that of brass makes a leakproof joint between the two difficult to achieve. Glass is the simplest material to use because the pieces can be ground to fit. Perhaps litharge and glycerin would make a good cement. Few liquids dissolve it.

"Those who wish to avoid the labor of building the two small telescopes used in this instrument may buy a pair of the popular-priced, low-power telescopes now on the market. I have a small spectroscope that uses a telescope and collimator with apertures of only three quarters of an inch. It will not separate Fraunhofer's D line of sodium but will resolve the yellow lines of mercury.

"Incidentally, Bunsen's way of rotating the prism to scan the spectrum makes you shudder. Of course he knew no better. Using fixed telescopes, he had to set the angles in a way which did not allow him to focus on any part of the spectrum at minimum deviation. Thus all measures of angles were made from an arbitrary, nonreproducible point. When provision is made for rotating the telescopes around the prism, the point of minimum deviation for a single line in the spectrum is easily found. Then rotation of one of the telescopes can be measured from such a position and a reproducible measure made.

"An easy way to make a good slit is with safety-razor blades. A thin, double edged blade snapped in two in the middle gives a fine pair of jaws. It is a painstaking job to file a pair of slit jaws, even as short as a quarter inch. The slit should be only two or three thousandths of an inch wide if you want to see detail.

Not all amateurs who go in for astronomy enjoy the challenge of ultra-precise craftsmanship involved in making a telescope or can afford to spend the necessary time. Consequently some of them simply buy a commercial telescope; others purchase the optical parts-oculars, mirrors, objective lenses and prisms-and assemble them in a homebuilt mounting.

One of the latter is Philip Lichtman, a high-school senior of Washington, D. C. Before he was 16, he demonstrated a remarkable talent for designing good mounts and making them of war surplus parts plus odds and ends.

"I have been interested in astronomy for as long as I can remember," writes Lichtman, "but I did not undertake the construction of a telescope until four years ago. After studying Amateur Telescope Making and Amateur Telescope Making-Advanced, I mounted a three-inch mirror acquired through a supply house. This project introduced me to the tricks and frustrations of such jobs as machining ball-bearing races for axis housings, turning tapered shafts from bar stock, indexing setting circles and so on. It also taught me how to adapt an ambitious design to the limitations of an inexpensive six-inch lathe.

"Since the instrument was to be used for photography, it was provided with an equatorial mounting, setting circles and an electric drive. Clamps were provided for both right ascension and declination. The system was wired to provide controlled variation of the speed in right ascension, and the slow motion in declination could be regulated manually. Guiding was effected at the eyepiece through a push-button control box held in the hand. Besides the three-inch mirror and its optical train, the tube also carried a 1.6-inch finder with coated objective and illuminated cross-hairs and a two-inch astrocamera adapted for cut film 2 1/4 by 3 1/4 inches.

"It soon became obvious that the camera was much too slow (f/12) for nebular photography. A search of

the secondhand market finally turned up a four-inch Zeiss Tessar photographic objective with a speed of f/5. This was set up for 8-by-10-inch plates, but it weighed far too much for the little three-inch mounting. Because my lathe would not take large parts, hard maple was selected as the basic material for a new and heavier mounting. The axes chosen for this job were saw-arbor shafts about 1 1/4 inches in diameter. They turned in babbitt bearings. This mounting was intended exclusively for photography and hence required a precise drive. I used a synchronous motor with fast and slow motions derived from a variable-speed motor through a train of differential gears [see drawing on page 124]. With the exception of the gears, the entire assembly was homemade. The arrangement worked perfectly and enabled me to make very 'tight' exposures. A variable-speed, reversible declination drive was then added to correct differential refraction and misalignment of the polar axis during long exposures, as well as for following the moon in declination. Guiding was accomplished at a magnification of 150 diameters with a three-inch, coated war-surplus achromat of 25 inches focal length and a Goodwin-Barlow lens. Two pedestals were made for this instrument: one a six-inch water pipe filled with and set in concrete in my back yard; the other, a portable affair with four legs."

A version of this equipment as modified by Lichtman for the solar eclipse of last June is shown on page 123. He built all of the cameras and arranged two of them to operate automatically from a home-built four-channel recorder [drawing on the opposite page].

"The recorder," he writes, "triggers the two cameras independently and registers the time of exposure. The third channel records manual determinations of eclipse contact times and the fourth records time signals broadcast from the Bureau of Standards radio station WWV. The cameras are tripped by solenoids energized by a built-in timing motor operating at one revolution per minute.

"After a demonstration of the completed rig, my father agreed to haul it and me to Keweenaw Peninsula in Michigan for the total eclipse last June. Everything was set up and fully tested the evening prior to the big event. You can guess what happened the next morning. The sun rose behind one of the densest fogs ever observed on the Peninsula! The paper tape was so soggy it refused to go through the recorder- even for a demonstration.

"The investment of time and labor was not a total loss, however. I entered some of my work in a national science contest for high-school students and fortunately won a cash award sufficient to pay for one of the exquisite mirrors by the California optician Thomas Cave. It is an f/8 of eight inches aperture and what appears to be a perfect figure.

"The design of an appropriate mounting for this mirror was undertaken immediately. It would be my last major project until after college years, so I decided to go all out. A mirror of this size and speed represents a pleasing compromise for amateur work: it is not too slow for nebular photographs nor too fast for point objects; the plate scale is large enough to show fine detail in photographs made at the prime focus but not so large that the image suffers greatly from bad seeing. The focal length of approximately 65 inches is sufficient for planetary and lunar photography. I decided to equip for all of these types of photography.

"Since I was unable to machine axes larger than those used in the eclipse camera, I decided to limit all overhang to a minimum and make do with the saw-arbor shafts. Wood was too weak for the type of mounting needed, and welded plate was too expensive. Happily someone suggested aluminum castings. As things turned out, they proved to be less expensive than any material investigated. So the mount was designed around ribbed castings made from my own patterns.

"The project took all my spare time for a considerable part of a year. But when it was completed, I owned a telescope which makes celestial photography a real pleasure [see bottom photograph at left]. It has electric drives in both right ascension and declination, setting circles that can be read direct to two minutes and half of one degree respectively, push-button control of slow motion, tight clamps, and even little flashlights for lighting the setting circles! The tube is of rolled aluminum an eighth of an inch thick, welded at the seam. Aside from the optical parts and rough castings, this weld was the only job in the instrument done by a professional. For adequate ventilation the tube was made three inches larger in diameter than the mirror. Its exterior was painted white and the interior was lined with sheet cork and finished in flat black. A small fan near the mirror ventilates the inside before a night's work begins. It vibrates too much, however, to be used continuously during the observing.

"The mirror cell is a three-point flotation system. It consists of three radial arms spaced at 120-degree intervals. The system is adjustably mounted to the tube. The mirror rests on three steel balls embedded in the radial members and is held in place by arms projecting from them.

"By providing both coarse and fine rack-and-pinion focusing, the design anticipated the use of oculars and cameras of widely varying focal length. The diagonal spider is a single-arm affair supported by the ocular carriage, so that it can move longitudinally with respect to the tube. The diagonal is rotatable so that light may be deflected either to the eyepiece or to a Newtonian-focus camera mounted at about 90 degrees with respect to the eyepiece.

"The finder is a three-inch f/5 coated surplus objective equipped with illuminated cross-hairs and a wide-field Erfle ocular. The guide telescope, a 4 3/8-inch refractor, has rack-and-pinion focusing and a movable tailpiece so that it may be set on a bright star slightly outside the limited field of the reflector. A cross-ruled illuminated reticule simulates the field of view of the reflector when used at the extremely high plate scale necessary for planetary photography. This refinement enables me to put nine images of Jupiter on each sheet of the 2 1/4-by-3 1/4 cut film.

"The long-focus reflector is supplemented by an f/2.5 Aero-Ektar camera of seven inches focal length, by a new housing for the four-inch Tessar and by two multipurpose units One of these is a projection camera which can increase the reflector's equivalent focal length to 240 feet. The other is a simple plate holder which may be inserted in the eyepiece adapter of any standard telescope.

"Like city-bound amateurs everywhere, we in Washington are plagued by a brilliantly lighted night sky. Blue exposures made with the Tessar objective and Eastman spectroscopic plates fog almost black in 30 minutes. I doubt that there is a real cure for the trouble. I have succeeded in punching through the glow some 200 times, however. Sometimes I use filters which transmit the hydrogen alpha line and block out city lights."

Bibliography

EXPERIMENTAL SPECTROSCOPY. Ralph A. Sawyer. Prentice-Hall, Inc. 1944.

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

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