122 THE POPULAR SCIENCE MONTHLY. 



outer aspects from one another ; yet they not only pursued their 

 great works in common, but also lived their daily social life to- 

 gether. They took walks in company in the environs of Heidel- 

 berg, 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 constit- 

 uents. Wollaston had, in 1802, discovered the dark lines in the 

 spectrum ; and Joseph Fraunhof er 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 Kirchhoif, 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 po- 

 larity, 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 ab- 

 sorb rays only of the same wave-length. Under this law conclu- 

 sions can be drawn from the dark lines of the solar spectrum 

 concerning the constitution of the sun's atmosphere. Kirch- 

 hoff '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 spec- 

 trum, 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 peram- 

 bulator 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 scien- 



