Such an amateur is R. M. Watrous, M.D., 1924 Flora Place, Highland Park, Ill., and his account, written by request, follows:

"The concave grating spectrograph is undoubtedly the simplest of all types for the amateur to construct because it has only one optical surface, requires no collimating or objective lenses, and can be constructed to give almost any desired amount of dispersion. The essential optical part can be bought for $4.50 and up; the rest of the instrument for the most part can be made of wood and scrap brass, and does not require elaborate machining. Scientific supply houses, such as the Central Scientific Co., Chicago, list in their catalogues a variety of replica gratings made by taking collodion impressions of famous gratings ruled on glass. These are offered in three grades at three sets of prices, depending on the degree of faithfulness with which the original rulings are reproduced. The medium grade has been found to give amply good performance in the instrument to be described. Replica gratings can be obtained mounted on silvered spherical concave mirrors of various focal lengths, and one of these is the only optical part needed for a concave grating spectrograph.

"The concave grating spectrograph may have various forms, but all are dictated by the optical principle laid down by Rowland, which states that the slit, the grating, and the spectrum must all three lie on the circumference of a circle whose diameter is equal to the radius of curvature of the mirror on which the grating is mounted. The mirror and the spectrum must lie at opposite ends of one diameter of the circle, in order to have images of the slit in focus, so the only possible variations in design must he obtained by changing the location of the slit along the circumference of the circle. In practice, the parts are arranged as shown in Figure 1.

"In order to understand how images are formed by a concave grating, it is worth while to imagine the mirror on which the replica is mounted set up facing the observer as for a Foucault test, using a slit at the center of curvature as the source of light. Under such circumstances, the image of the slit will be cast back upon it, and a percentage of the light striking the mirror will be returned. However, the grating will cause some of the light to be diffracted, with the result that varicolored images of the slit will be formed both to the right and to the left of the direct image Thus there are two first-order spectra. To the right and left of these respectively will be another set of colored images, forming the second-order spectra, and beyond these will be third-, fourth-, and nth order spectra, each fainter and more spread out than the last. If the slit be now moved to one side sufficiently, and also be brought closer to the mirror, to preserve the relationships of Rowland's circle, the direct image of the slit will move in the opposite direction, as will all the spectra until a position is reached in which the images will lie in the positions shown in the figure. By moving the slit still farther, the second-order spectrum could be brought opposite the mirror. [Detail in extension of this particular paragraph, by Dr. Watrous, may be borrowed from this department on request by readers interested in this type of spectroscope.-Ed.]

"Having determined the dimensions required to secure the relationship shown in the figure with any given concave grating, it is necessary only to construct a box to support the parts and exclude light, and one has a spectrograph. A film-holder may be made, to support films at the point where the spectrum comes to focus, or a telescope eyepiece may be supported at this point for direct observation. With such an instrument, clear spectrograms may be obtained 8" long, showing hundreds of details such as Fraunhofer lines. The light obtained by sparking two iron nails across the terminals of a storage battery will give a beautiful line spectrum of iron. Light passed through solutions of hemoglobin, dyes, chlorophyll, and others, will show characteristic absorption bands. Bunsen's famous experiment with the sodium flame can be performed; and so on.

"In choosing a grating, a few facts should be kept in mind. The degree of dispersion (and thus the length of the first-order spectrum) is proportional t the number of lines per inch in the grating and to the focal length of the mirror. The resolving power, however, depend on the area of the grating and the accuracy of the ruling. The spectrograph shown (Figure 2) contains a medium grade grating of about 40" radius of curvature, with 14,500 lines to the inch an a grating area of about 2 x 3 centimeters.

"The box is made of plywood, painted black inside, and with light baffles locate at strategic points. The adjustable slit mechanism is on the left. The sliding adjustment for the film-holder is on the right, actuated by the two vertical screws In Figure 2 the film-holder has been removed and is resting on top of these screws, its slide pulled two thirds of the way out to show how it is loaded. The main body of the camera extends into the background, with a small square porthole in the far end to give access to the adjustments of the grating mount. [In Figure 1 the slit is on the right of the spectrum but it matters not.-Ed.]

"The slit of a spectroscope is one the essential parts, since every detail the spectrum is actually an image of slit. Its edges should be as smooth and as parallel as the maker's skill can contrive, and one may lavish as much or as little care on it as he wishes. For ordinary work, a slit 0.002" to.0.003" in width is suitable, and there is very little actual use for an adjustable width. Provision should be made for rotating the slit mounting to line it up parallel with the ruling of the grating for best definition, and it should be mounted in a draw-tube so that its distance from the mirror can be varied slightly for focus.

"Thirty-five millimeter film is very useful for making spectrographs, and a holder may readily be designed to accommodate strips long enough to take in the full length of the spectrum. The film should be held in a curve conforming to Rowland's circle. Though it adds considerably to the problems of construction, a slide which permits the film holder to be moved at right angles to the length of the spectrum in the same plane will prove well worth while, since it allows up to ten spectra to be made on one film, with all the advantages of being lined up for comparison with one another. With such an arrangement, a narrow slit-like mask should be placed just in front of the focal plane so that each spectrum occupies a strip about 2 mm wide running the length of the film.

"The grating mount, as all telescope makers will realize, must be adjustable as to tilt in two dimensions, and it must have an adjustment for rotation about the mirror axis, in order to line up the rulings in a vertical position.

"Light sources are many and varied, the ordinary incandescent filament lamp gives a disappointing spectrum. Sunlight, with its thousands of Fraunhofer lines, furnishes material for many hours of study, but one must secure the co-operation of some patient soul to wield a mirror in order to direct the light into the slit, or else make a heliostat. If photographs are made, of course, they can be studied at any time.

"Carbon arcs give fine line spectra, and are excellent for collimating the instrument. Cored projection carbons, 5/16" size, will operate well on ordinary house current if the arc is placed in series with a cheap heating element or electric iron drawing about 500 watts. If the carbons are removed from this circuit and replaced by iron nails, copper wires, aluminum, brass, nickel, or lead rods, light can be produced by making and breaking the contact, which will give beautiful brightline arc spectra of the metals. These flashes are too fleeting to study visually, but can be recorded on film and the lines can be identified with the aid of a table of wavelengths.

"If the experimenter has a small transformer, such as is used for neon signs, he can obtain spark spectra of metals by causing the spark to jump between electrodes of the proper material. In order to obtain emission of lines, however, a condenser must be placed across the secondary of the transformer. The writer found that a home-made, one-quart Leyden jar served this purpose very well, though a more efficient and less bulky condenser would be preferable.

"The light from neon signs, fluorescent lights, and sodium vapor lamps will furnish interesting material for study and will also challenge the ingenuity of the experimenter to find some way of making it enter the instrument. On one occasion I balanced my spectrograph, which is about the size and shape of a baby's coffin, across the back seat of my car, while my wife sighted it like a rifle at a sodium vapor lamp and I held a condensing lens so as to cast an image on the slit. This was on a busy highway, but fortunately the stunt took place at night!

"Figure 3 shows three contact prints made from portions of negatives obtained in the instrument. They extend from the orange to the violet. Violet is on the right.

Top. Arc and spark spectra of metals; from top to bottom: aluminum arc; 15,000-volt condensed spark between A1 electrodes; iron arc; spark between iron electrodes; copper arc; copper spark; nickel arc; nickel spark; spark between platinum electrodes.

"Middle: Series of exposures made with carbon arc for purpose of focusing slit.

"Bottom: Absorption spectra of hemoglobin derivatives: carbon arc; next two, light from incandescent portion of carbon arc passed through hemoglobin solutions; next two, same through methemoglobin solutions; next two, same through carbon-monoxide-hemoglobin (note shifting of the, two dark bands to the right); carbon arc.

"The two D lines of sodium are especially prominent in most of the negatives made with light from the carbon arc.

They appear near the left margin in middle set of spectrograms, and serve convenient landmarks.

"Brode's 'Chemical Spectroscopy,' 1939, is a fine technical work with 120-page tables of wavelengths and other things but no information on building spectroscopes. The old maestro of the subject was Baly, whose 3-volume 'Spectroscopy' was published in 1912, but this, too, is pretty technical and is no longer in print."

STELLAFANE convention of amateur astronomers and telescope makers has been called off this summer. War.

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/

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