NATURAL PHILOSOPHY. 165 



Kirchhoff's purpose was not merely to observe the fine vertical 

 dark lines which in untold numbers crossed the colored spectrum, 

 stretching from right to left ; he wished to measure their relative dis- 

 tances, and thus to map them, exactly as the astronomer determines 

 the position of the stars in the heavens, and the surveyor triangulates 

 and marks out the main features of a country, so that future wander- 

 ers in this new field may find fixed and well-recognized points from 

 which to commence their own excursions. Professor Kirchhoff is far 

 from thinking that his measurements, delicate and numerous though 

 they be, have exhausted the subject. The further we penetrate into 

 the secrets of nature, the more we find there remains to be learned. 

 He saw whole series of nebulous bands and dark lines which the 

 power of his instrument did not enable him to resolve ; and he thinks 

 that a larger number of prisms must be employed to effect this end. 

 He adds : " The resolution of these nebulous bands appears to me to 

 possess an interest similar to that of the resolution of the celestial 

 nebulas, and the investigation of the spectrum to be of no less im- 

 portance than the examination of the heavens themselves." True, 

 indeed, does this appear, when we learn that it is by the examination 

 of these lines that we can alone obtain the clue to the chemical com- 

 position of sun and stars ! 



The exact measurement of the distances between the lines was 

 made by moving the cross-wires of the telescope from line' to line by 

 means of a micrometer screw with a finely divided head, and reading 

 off the numbers of divisions through which the screw had to be turned. 

 The breadth and degree of darkness were also noticed, and thus the 

 lines were mapped. In order to give a representation in the drawing 

 of the great variety of the shade and thickness of the lines, they 

 were arranged according to their degree of blackness, and drawn o*f 

 six different thicknesses. First, the darkest lines were drawn with 

 thick, black Indian ink ; the ink was then diluted to a certain extent, 

 and the lines of the next shade drawn, and so on to the lightest 

 series. As soon as a portion of the spectrum had been drawn in this 

 manner, it was compared with the actual spectrum, and the mistakes 

 in the breadth and darkness of the lines, as well as in their position, 

 corrected by fresh estimations, and the drawing made anew. A 

 second comparison and another drawing were then made, and this 

 process repeated until all the groups of lines appeared to be truthfully 

 represented. In the English edition of Kirchhoff's Memoir, above 

 referred to, lithographic representations of the appearance of the 

 spectrum through Kirchhoff's instrument are given, presented in six 

 different colored tints. 



In the Annual of Scientijic Discover)/ for 1862, it was shown how 

 an examination of these dark lines of the solar spectrum had revealed 

 somewhat of the chemical constitution of the solar atmosphere, and 

 also how the existence and development of the lines had been ap- 

 plied for the chemical analysis of terrestrial matter. In regard to 

 the latter point it is unnecessary to say more than that Mr. Kirchhoff 

 announces in his Memoir that he has carried spectrum analysis to 

 such perfection that he has without difficulty been able to distinguish 

 the presence of minute traces of the rare metals erbium and terbium, 

 as well as cerium, lanthanum, and didymium, Avhen the}- are mixed 



