The top spectrogram was made with light from an arc between ordinary cored carbons such as are used for some kinds of lantern slide projectors. Those below it were obtained by melting bits of common metals (tin can strip, tinfoil, solder, galvanized iron, brass, copper, sterling silver, silver solder, aluminum, iron) in the heat of the arc, thus causing them to vaporize and emit their characteristic wavelengths. Some of these metals did not remain in the arc long enough to record their spectra.

Since the carbons used were not of the high degree of purity required for spectrographic analysis, the lines of the elements in the core (silicon, bismuth, and magnesium) appear in all of the spectra, and are indicated at the top by their chemical symbols. The other numbers written across the top refer to the wavelengths in angstroms.

IN THE concave grating type of spectroscope mentioned above and described last month, the slit, the grating, and the spectrum, all three, lie on the circumference of a circle whose diameter is equal to the radius of curvature of the grating. How a really large-gigantic, in fact-instrument of this type shapes up is shown in Figures 2 and 3. This is the world's second largest spectrograph, 30 feet in diameter, and is at the University of Chicago. It took over a year to build and in size it is rivaled only by a similar spectrograph at the Massachusetts Institute of Technology. The diffraction grating, in addition to focusing the light breaks it up into its component wavelengths and fans it out over the 40' of photographic plate.

The physics department at the University of Chicago began making diffraction gratings under the supervision of the late Dr. Albert Michelson, and after Dr. Michelson's death, in 1931, the work was continued under Dr. Henry Gordon Gale. From five to ten of these gratings have been made every year by the University and sold to scientific laboratories in all parts of the world. Only at the Johns Hopkins University are gratings made which compare, in their power to separate the spectrum lines, with those made at the University of Chicago. The grating consists of an octagonal piece of speculum metal on which vertical lines have been ruled by a ruling engine, the time required being approximately two weeks.

The giant spectrograph fills a room 40' square. A light beam from the compound to be studied originates in apparatus outside the room, passes through a narrow slit at the point in the photograph (Figure 3) to which Professor Robert S. Mulliken is seen pointing, travels across the room, strikes the grating at which Miss Jane Hamilton, a graduate student in physics, gazes (this grating is shown in Figure 2), and then is reflected to different parts of the big circle as a large fan of light. This light impinges on the photographic plates in a strip 2" in width a 40' in length, arranged in a large semicircle 30' in diameter. (The inner circle in the right half of Figure 3 is the vestige of an older and smaller spectroscope of the same type.) The photographic plate is composed of 26 sections, each 18" in length. The sections are removed separately for developing in the neighboring darkroom. It takes from three to 40 hours to secure an adequate exposure because the dispersion of the light greatly diminishes its intensity. If an observer stands within the fan of light coming from the grating, and walks to different parts of the room, he can witness the differences in wavelengths of the light in various parts the fan. At one side of the room long wavelengths will cause the grating to appear a brilliant scarlet. As the observer moves across the room the color of the grating passes from red through the seven colors of the spectrum to violet.

SLIT for spectroscopes or other optical instruments was described by Dr. John Strong, of the Astrophysical Observatory, California Institute of Technology, in The Review of Scientific Instruments, Vol. 12, pages 213-214, as follows:

"Many types of slits have been used or optical instruments. (See H. Kayr, Handbuch der Spektroscopie Vol. 1, p. 532). Among them one bilateral slit which we may term the parallelogram slit, and which is diagrammatically illustrated, in Figure 4(a), has several noteworthy features. When it is skillfully constructed, this slit is both simple and effective. The opening of the parallelogram slit exhibits an adverse non-uniform relationship to the amount of turn of the adjusting screw: The slit opening changes most rapidly when the slit is nearly closed whereas we would prefer, to have a more delicate control, that the slit opening change slowly when the slit is nearly closed and rapidly when it is nearly open. Figure 4(b) represents diagrammatically the principle of the parallelogram slit adapted to achieve this desired end and Figure 5 illustrates how we apply this principle in practice.

"Figure 5 is a sketch by Mr. R. W. Porter drawn from one of the slits of a new spectrometer under construction here. The cover plate, with the slot for illuminating the slit, is shown turned to one side. This plate is secured by screws in the four corners while the slit, as a whole, is fastened to the spectrometer by four other screws, two on the right side and two on the left. "The micrometer screw displaces the slit jaw assemblies equally in a direction parallel to the slit and, by virtue of their 0.006" clock spring mountings, this displacement causes the slit jaw assemblies to separate and the slit to open. Two helical springs locate the jaw assemblies definitely against the hardened end of the micrometer screw. The carefully worked slit jaws are adjustably fastened to the jaw assemblies so that the jaws will close exactly.

"The advantages of this type of slit over other types are: that the jaws cannot possibly be jammed; that the slit opening is delicately controllable when the slit is narrow; and the spring mounting, as contrasted with mounting in ways, provides a definitely reproducible mechanical system. Disadvantages are: the non-uniform relation between micrometer screw setting and slit opening and certain limitations on compactness of construction. The slit is relatively easy to construct in such a manner as to yield high accuracy.

"The slit opening is given, approximately, by the expression:

S = 2L [1 - cos (M/L)], where L is the free length of the clock springs and M is the displacement of the micrometer screw from the closed position. For the slits already constructed L = 1 cm and M = 3 mm giving S approximately 1 mm. The slit jaws are 24 mm long while the useful length of slit is 12 mm. The over-all dimensions of the slit exclusive of the micrometer head are 3 1/2" x 3" x 3/4"."

ANOTHER spectroscope-in fact, a spectrometer-made by an amateur is shown in Figure 6. F. P. Smith, Box 364, Ventura, California, is the maker, and he says he has followed spectroscopy as a hobby for 16 years and found it to have unlimited fascination and possibilities. "The concave grating type is the easiest," he writes, "also the most practicable to build. Gratings in the original can be obtained with a 2" ruling and 25,000 lines per inch at $50 to $150. About $75 will build an instrument having excellent dispersion. A very good slit can be made without machine tools, the practice it gives in filing also being of value.

"In the photograph (Figure 6), the screw which is used to oscillate the cylinder, or drum, is grooved and the groove moves the eyepiece across the spectrum as the hand-screw is turned. The spiral strip on the drum is calibrated in angstrom units; it also carries the symbols of the principal lines of the elements. Thus, if 15 or 20 elements are present, the user gets a line on the crosshair and then refers to the drum.

"The camera part is at the extreme right. The round white spot is the hand-screw which is used to oscillate the grating."

Smith's description was shown to Watrous, who replied: "This apparatus is undoubtedly more accurate than mine though probably more difficult to build. The more expensive gratings must give much better definition."

CONTACT: I have long ago abandoned the pencil mark system measuring contact between mirror and tool," Cyril G. Wates of Edmonton, Alberta, Canada, writes, "not because of the alleged danger of scratches, but because it doesn't work-at least, not for me. So I took a leaf from the mechanic's book-he uses Prussian blue in making face plates-and put a streak of dry rouge on the tool, and judged contact by rubbing out of the streak, but by the rouge which was transferred to the mirror. This, as I mentioned in 'A.T.M.A.' works well, but I have found, especially, with Pyrex, that the faint rouge marks are difficult to distinguish.

"Recently I discovered a very satisfactory substitute in artist's Black Stumping Chalk. This is a very fine black powder-probably pure carbon-and a little may be taken on the finger tip rubbed across a diameter of the tool. After dusting off the loose particles, mirror is pressed into contact and rubbed gently, without pressure. When the mirror is removed, the points of contact show on the face of the mirror like an Ethiopian on a snowbank."

Suppliers and Organizations

The Society for Amateur Scientists
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Internet: http://www.sas.org/

At Surplus Shed, you'll find optical components such as lenses, prisms, mirrors, beamsplitters, achromats, optical flats, lens and mirror blanks, and unique optical pieces. In addition, there are borescopes, boresights, microscopes, telescopes, aerial cameras, filters, electronic test equipment, and other optical and electronic stuff. All available at a fraction of the original cost.

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