In addition to the seven colours of the Newtonian spectrum, Sir John Herschel has discovered a set of very dark red rays beyond the red extremity of the spectrum which can only be seen when the eye is defended from the glare of the other colours by a dark blue cobalt glass. He has also found that beyond the extreme violet there are visible rays of a lavender gray colour, which may be seen by throwing the spectrum on a sheet of paper moistened by the carbonate of soda. The illuminating power of the different rays of the spectrum varies with the colour. The most intense light is in the mean yellow ray, or, according to M. Fraunhofer, at the boundary of the orange and yellow.
When the prism is very perfect and the sunbeam small, so that the spectrum may be received on a sheet of white paper in its utmost state of purity, it presents the appearance of a riband shaded with all the prismatic colours, having its breadth irregularly striped or subdivided by an indefinite number of dark, and sometimes black lines. The greater number of these rayless lines are so extremely narrow that it is impossible to see them in ordinary circumstances. The best method is to receive the spectrum on the object-glass of a telescope, so as to magnify them sufficiently to render them visible. This experiment may also be made, but in an imperfect manner, by viewing a narrow slit between two nearly closed window-shutters through a very excellent glass prism held close to the eye, with its refracting angle parallel to the line of light. The rayless lines in the red portion of the spectrum become most visible as the sun approaches the horizon, while those in the blue extremity are most obvious in the middle of the day. When the spectrum is formed by the sun’s rays, either direct or indirect—as from the sky, clouds, rainbow, moon, or planets—the black bands are always found to be in the same parts of the spectrum, and under all circumstances to maintain the same relative positions. Similar dark lines are also seen in the light of the stars, in the electric light, and in the flame of combustible substances, though differently arranged, each star and each flame having a system of dark lines peculiar to itself. Dr. Wollaston and M. Fraunhofer, of Munich, discovered these lines deficient of rays independently of each other. M. Fraunhofer found that their number extends to nearly six hundred, but they are much more numerous. There are bright lines in the solar spectrum which also maintain a fixed position. Among the dark lines, M. Fraunhofer selected seven of the most remarkable, and determined their distances so accurately, that they now form standard and invariable points of reference for measuring the refractive powers of different media on the rays of light, which renders this department of optics as exact as any of the physical sciences. These lines are designated by the letters of the alphabet, beginning with B, which is in the red near the end of the spectrum; C is farther advanced in the red; D is in the orange; E in the green; F in the blue; G in the indigo; and H in the violet. By means of these fixed points, M. Fraunhofer has ascertained from prismatic observation the refrangibility of seven of the principal rays in each of ten different substances solid and liquid. The refraction increased in all from the red to the violet end of the spectrum. The rays that are wanting in the solar spectrum, which occasion the dark lines, were supposed to be absorbed by the atmosphere of the sun. But the annular eclipse which happened on the 15th of May, 1836, afforded Professor Forbes the means of proving that the dark lines in question cannot be attributed to the absorption of the solar atmosphere; they were neither broader nor more numerous in the spectrum formed during that phenomenon than at any other time, though the rays came only from the circumference of the sun’s disc, and consequently had to traverse a greater depth of his atmosphere.