Popular Science Monthly/Volume 15/October 1879/A Home-Made Spectroscope
By JAMES J. FURNISS.
THE person to whom the study of spectroscopy is really attractive and congenial will not rest satisfied with mere reading, but, sooner or later, will experience a desire to possess a spectroscope of his own—to see for himself the phenomena which are described in the books. He who possesses and can spare the requisite means, will naturally provide himself with an instrument from the optician; but there are no doubt many who, while taking a great interest in this and kindred subjects, are so circumstanced that their outlay for scientific purposes must be limited to a very small sum. It is hoped that this article may be of some service to readers whose fortune places them in the latter category. I do not intend to say anything concerning the principles of spectrum analysis, or the construction and use of spectroscopes in general; that part of the subject may be studied in such treatises as "The Spectroscope and its Applications," by J. Norman Lockyer, or "The Spectroscope and its Work," by Richard A. Proctor, as well as in the more advanced works by Lockyer, Roscoe, and Schellen. I simply propose to give a few hints (which the works mentioned do not give), to enable the beginner, though he may possess little or no mechanical ingenuity, to construct at small expense an instrument which will prove a useful adjunct to his studies.
The chief quality to be desired is usefulness; the appearance of the instrument counts for little: if its performance be satisfactory, that is all that is necessary. The essential parts of the spectroscope are, (1) the prism, (2) the collimator, (3) the telescope, and (4) the stand. The prism is the most important part of the instrument, and also the most expensive; but, as so much depends upon the performance of the prism, a good one obtained at the first will prevent the disappointment which inevitably follows the attempt to use a cheaper and less perfect article.
A hollow prism filled with a liquid such as oil of cassia, or bisulphide of carbon, may be used, or a flint-glass prism, or one of crown glass. A crown-glass prism may be procured for a comparatively small sum, but its dispersion is small, and to obtain really satisfactory results a train of three or more prisms is necessary, and such a number would be difficult for the beginner to handle without the automatic arrangement to be found in the regular instruments. The price of the hollow prism is a little higher than that of the crown-glass prism, but not quite so high as the price of one of flint glass; of its performance I know nothing by experience, but Lockyer does not speak very favorably of it. Perhaps on the whole the most satisfactory, and consequently in the end the cheapest article, is the flint-glass prism.
The collimator is a tube carrying at its outer end the slit, and at the end next the prism the collimating lens. The tube should consist of two pieces, one sliding easily within the other; so that the distance of the slit from the lens may be regulated. That distance should be equal to the focal length of the lens, in order that the rays of light passing through the slit in diverging pencils may be rendered parallel and sent through the prism as a cylindrical beam. Fig. 1 shows the collimator—A being the position of the slit, and B the position of the lens.
The focal length of the lens may be obtained near enough to give the approximate length of the collimator tube by projecting the image of some distant object sharply and distinctly on a screen, and then measuring the distance between the screen and lens. After the instrument is completed the adjustment of the lens and slit with regard to each other may be perfected by the following operation: Remove the prism, and bring the telescope, which must previously have been focused for distant objects, into line with the collimator; then move the sliding tube carrying the slit in or out until the image of the slit is seen sharply defined in the field of view of the telescope. The distance then between the slit and lens is equal to the focal length of the lens. The diameter of the lens need not be quite equal to the width of the refracting face of the prism. An ordinary convex lens of eight or ten inches focus, which may be purchased for a small sum, and which may easily be set in the tube by the student himself, will answer his purpose as well as a much more expensive article.
The figure of the slit is of great importance, and for fine work a finished piece of mechanism with adjusting screw, etc., is necessary; but the beginner may content himself with an arrangement of much humbler pretensions. I have used a small plate of silver in which an opening (about three eighths of an inch long and one twentieth of an inch wide) had been cut, the width of the opening being reducible at pleasure by means of a little door or shutter sliding smoothly in grooves, and each opposing edge being faced with a strip of watch-spring. Fitted closely into the end of the tube is a piece of cork or wood having in its center a hole larger than the aperture in the slit plate; over this hole the slit plate is fastened, care being taken to stop up any accidental holes or crevices in the cork (or wood, as the case may be), in order to prevent the entry of extraneous light into the tube.
Primitive as this contrivance may appear, I have seen by its means, when using two flint-glass prisms, the D line of sodium double, and beautifully distinct. A slit, such as I have described, need not cost more than one tenth of the price charged for a slit by the optician.
Concerning the observing telescope, perhaps Proctor's remarks about a finder for an astronomical telescope may be repeated: "It will be easy for the student to construct one for himself, and will be a useful Fig. 2. exercise in optics." But in case the student may not want to take the trouble, he will be glad to remember that an ordinary pocket telescope or spy-glass may be purchased ready made for a dollar or two, and by removing the erecting lenses a small astronomical telescope may be produced which, as the magnifying power required is small, will answer every purpose of the beginner.
The stand may be made in a variety of styles, from the unpretending box on legs, with holes in the sides for the collimator and telescope, to the highly finished tripod of the most aristocratic looking instrument. The following is one way of making a cheap and at the same time serviceable stand: Procure two disks of seasoned walnut or mahogany or any other hard wood, one about a foot in diameter by three eighths of an inch in thickness, and the other six inches by one quarter of an inch; also get two strips of wood about eight inches long and an inch and a half wide.
Make the larger disk into a table by screwing on three feet—metal hooks such as are used for hanging up clothes make excellent feet; then make a hole in the center of the large disk, and a corresponding hole in one end of each of the strips. Pass a screw downward through the two strips and through the hole in the disk, and let there be a thumb-nut on the screw so that it may be tightened underneath. Fasten the lower strip of the two permanently to the disk, leaving the other strip free to move in an horizontal plane about the center of the disk. You will then have a stand like that represented in Fig. 3,
A being the stationary arm, and B the movable arm. Make the small disk of wood also into a table (see Fig. 4) by fastening in three sharp—feet screws will do. Place this smaller table centrally on the larger Fig. 4. one, and it becomes the platform on which the prism is to stand. The next thing is to put the collimator and telescope in their places. For this purpose two small oblong blocks of wood are needed, each one having the upper part hollowed out into a groove to take—one the collimator, the other the telescope. (See Fig. 5, d, d.) The block carrying the collimator is placed on the stationary arm, and the one with the telescope on the arm which is free to move. Both are secured in their places by elastic bands. The blocks of wood must necessarily be of such a height that the axis of their respective tubes (collimator and telescope) may be brought into alignment (1) with
each other, and (2) with the prism. The instrument is then complete (see Fig. 5), where A is the collimator, B the prism, and C the telescope.
The manner of using the spectroscope has been described in such works as those I have already mentioned, and does not properly belong to an article such as this, but perhaps a few hints as to the adjustment of the instrument may not come amiss to the beginner. The angular position of the prism, with regard to the collimator, is a matter of importance, the distinctness and purity of the spectrum depending in a great measure on that position. Perhaps as good a way as any is to find by actual trial the angular position of the prism which gives the best results, thus: Focus the telescope for distant vision, align the telescope with the collimator, and move the slit till it (the slit) is seen distinctly in the telescope; then put the prism in place approximately, and move it round its axis until a position is found where (supposing the light of the sun or diffused daylight to be under examination) the lines in the green part of the spectrum are seen at their best advantage. It must be remembered that a slight change of focus is necessary for every color. When the bounding edges of the ribbon of variegated light which forms the spectrum are seen sharply defined at the same time that the lines across the spectrum are distinct, the adjustment is pretty correct; when such is not the case, the slit and lens are not in their proper relative positions. The narrower the slit, consistent with the necessary supply of light, the finer and more distinct are the lines. A black cloth, or a pasteboard box, with suitable apertures for the collimator and telescope tubes, should be placed over the prism to shut out all extraneous light. In conclusion, I must remark that I do not pretend to describe an instrument capable of doing exact or delicate work. My aim is simply to show with how little trouble or ingenuity a spectroscope may be put together which will help materially those students who wish to obtain a good general idea of a branch of science which has done more to unravel the mysteries of nature than any other inquiry with which the human mind has ever been occupied.