The advance of photography : its history and modern applications (1911)

306 THE ADVANCE OF PHOTOGRAPHY moon, and stars are not the only objects of astronomical photography. Its province extends further, since the discovery of spectrum analysis. When it was discovered that the wonderful lines intersecting the sun's spectrum were occasioned by glowing substances of different kinds, and that each element shows invariably the same lines, so that the presence of certain spectral lines establishes the presence of certain elements, it became necessary to possess an exact map of the countless lines of the solar spectrum. Then, by comparing this map with the spectrum of a flame or of a star, it could be at once seen what substances give these lines. KirchhofT, one of the discoverers of spectrum analysis, and Angstrom have prepared such maps of the solar spectrum. Their labour would have been materially simplified had Rutherford published his photograph of the spectrum a year earlier. Rutherford's photograph only shows the lines of the chemically active part of the spectrum — from green to violet — but it does this with wonderful clearness. Many lines that appear faint to the naked eye show themselves strong and sharp in the picture ; nay, lines are discovered in the photograph of the spectrum which Kirchhoff did not see at all in the spectrum itself. The causes of this phenomenon may be twofold : either the eye is not sensitive to certain rays of light — as we know it is not influenced by the ultra-violet rays, which have a strong photographic effect — or it is possible that changes take place in the sun : that at certain times fresh substances come to its surface, and thereby new lines become apparent. The photograph of the spectrum may be taken with the aid of an ordinary spectroscope, seen in fig. 128. This consists of a tube A, which has a fine slit F, through which the light penetrates, and at the other end a lens, which makes all the rays from the slit parallel, and con