Page:Popular Science Monthly Volume 33.djvu/132

122 outer aspects from one another; yet they not only pursued their great works in common, but also lived their daily social life together. They took walks in company in the environs of Heidelberg, and they traveled together during the vacations.

The discovery of the spectrum analysis is destined, like that of gravitation by Newton, and a few others, always to rank among the greatest achievements in the history of science. Newton had succeeded in separating white sunlight into its colored constituents. Wollaston had, in 1802, discovered the dark lines in the spectrum; and Joseph Fraunhofer had, independently, some ten years later, investigated those lines thoroughly, fixed the position of more than five hundred of them, and marked the principal groups with letters. Now, half a century afterward, Kirchhoff found the key to the remarkable phenomenon. For it is really the law of the relation of emission and absorption, as discovered by Kirchhoff, that furnishes the theoretical basis of spectrum analysis. More precisely expressed, this law declares that, for a given temperature and rays of the same color and polarity, the relation of the power of emission and absorption is the same for every body—that is, independent of the nature of the body. From this theorem it follows that a luminous body which sends out light-rays only of a certain wave-length, will also absorb rays only of the same wave-length. Under this law conclusions can be drawn from the dark lines of the solar spectrum concerning the constitution of the sun's atmosphere. Kirchhoff's first publication on this subject appeared in the monthly reports of the Berlin Academy for October, 1859. This short notice was followed by a rapid succession of papers describing the researches of the two investigators (Kirchhoff and Bunsen) upon the solar spectrum and the spectra of the elements.

While engaged in these investigations, Kirchhoff injured his vision by exposure to the glare of a clear spot in the solar spectrum, so that in later years he was obliged to spare his eyes. In 1867 he suffered an injury of his foot, in consequence of which he was for three years unable to get about except upon a perambulator or with the aid of crutches, and his health was affected for the remainder of his life. But the results of this personal mishap were seen in his physical life only, not in his labors.

In 1875 Kirchhoff accepted a call to the professorship of Mathematical Physics in the University of Berlin, after having previously declined two invitations to the same institution. Here he delivered for several years regular courses of lectures on the mechanics of solid and fluid bodies, the theory of heat and light, electricity and magnetism, mathematical optics, and special topics in hydrodynamics, electrodynamics, etc. “Whether life in Berlin,” says Robert von Helmholtz, “is favorable to