SPECTRUM (Gr.) is a term applied in optics to the colored image of the sun or other luminous body, produced by refraction through a prism' (q. v.), by disfraction (q.v.) through a fine grating, etc. In what follows we shall confine ourselves to the spectrum produced by a prism, as that which is commonly used. Besides, so far as we have at present occasion to examine it, it presents very nearly the same appearance as spectra produced by other processes.
The Solar spectrum was first carefully examined by Newton, who deduced from his observations the composite nature of white light, and the different rcfrangibilities of its various colored constitutents. A ray of sunlight enters a darkened room through a small hole, in a shutter. It proceeds in a straight line to the opposite svall, and forms as a circular white spot, an image of the sun. If the edge of a glass prism be interposed in the path of this ray, the white spot disappears, and the spectrum is produced. In this iman of experiment, its shape is that of 'a rectangle with semicircular ends.
The breadth of the spectrum is equal to the diameter of the spot; and it is brilliantly colored, the one end being red, and the other end violet. Between these we have grada tions of color, and the whole appeared to Newton to be divisible iuto seven differently colored spaces, which he called red, mange, yellow, green, blue, indigo, violet. /This was in accordance with the speculations of old days, when analogies were constantly looked for, and seems to have been suggested to Newton by the common musical scale. It is impossible, however, to settle precisely the exact boundary between any two of these fancied species of color; and, besides, such a description of the spectrum (though complete enough for mere popular language) is totally inadequate to express our present knowledge of the subject. In order to study the spectrum a little more closely, suppose we have pieces of colored glass which allow only one definite color to pass. With a red glass placed at the hole in the shutter, the prism being removed, the effect would be to render the spot red, without changing its position. Introduce the prism, and the effect is to change the position of the spot without altering its size or color.
Similarly, with a violet glass we have a violet spot, and so on; the change of position, due to refraction, being least for red and greatest for violet. It thus appears that the spectrum formed in this way, is made up•of a series of circular spots, of the various colors of which white light consists all of the same size, and having their centers ranged along a line, so that each overlaps those next it. The only parts of the spectrum which are pare, i.e., where uo two or more colors are mixed, are the ends; so that, by this process, it is impossible to separate definitely the rays of different refrangibility, so as to see, for instance, whether any are wanting. How, then, are we to ascertain whether sunlight contains rays of every refrangibility from red to violet? The obvious method is to make the spot not circular, but long and very narrow, a process mentioned by Newton himself. To make this spot thus narrow, a method commonly employed, is to set the prism abort half-way between the shutter and the screen and to place before it a Im.4, such that, if the prism were removed, there would be an image of the hole in the shatter nearly equal to it in size. The hole must, therefore, be a narrow slit, paral lel to the edge of the prism. When this arrangement is adjusted we have a pure spec trum, and we find it to be (at first sight) eontinaeas. Thus, it appears that sunlight con tains rays of every refrangibility, from the highest to the lowest; and that Newton's sevenfold division of it, though sometimes convenient for popular reference, has no scient.iti basis. Besides, what we can see is not the whole spectrum but a mere fraction , of it; for beyond the red end, there are invisible rays recognized at once by their heat ing powers; and beyond the violet, there are invisible rays more powerful than the visible in producing chemical changes, as on a photographic plate, and which can be changed into visible rays by fluorescent substances. See PHOSPHORESCENCE. The breadth of the visible spectrum evidently depends on the length of the slit, its length on the difference of refrangibility of red and violet.