and on those produced by the Earth's Atmosphere. 525 



bited a distinct coincidence with some of the principal dark lines 

 in the solar spectrum, and thus indicated that these lines marked, 

 as it were, weak points of the spectrum, on which the elements 

 of material bodies, whether they existed in the solar atmosphere 

 or in coloured solids and fluids, exercised a particular influence. 

 These actions, however, were so indefinite, that, with the excep- 

 tion of the oxalate of chromium and potash, a salt of most re- 

 markable properties, they never appeared in the form of lines or 

 distinct bands. The light which was left shaded into the dark 

 spaces, and therefore, notwithstanding the general coincidence 

 which I had observed, the phenomena of ordinary absorption 

 could not be identified with those of the definite actions by 

 which the solar lines are produced. 



This point of similarity, however, led me to institute a dili- 

 gent comparison between the solar lines and those of the nitrous 

 acid gas spectrum ; and it did not require many experiments to 

 prove, that there existed between these two classes of phenome- 

 na a most remarkable coincidence. In order to afford ocular de- 

 monstration of this fact, I formed the solar and the gaseous spec- 

 trum with light passing through the same aperture, so that the 

 lines in the one stood opposite those on the other, like the divi- 

 sions in the vernier and the limb of a circle, and their coincidence 

 or non-coincidence became a matter of simple observation. I 

 then superimposed the two spectra, when they were both formed 

 by solar light, and thus exhibited at once the two series of lines, 

 with all their coincidences, and all their apparent deviations from 

 it. Professor AIRY, to whom I shewed this experiment, remark- 

 ed, that he saw the one set of lines through the other, which is 

 an accurate description of a phenomenon, perhaps one of the most 

 splendid in physical optics, whether we consider it as appealing 

 to the eye or to the judgment. 



The general coincidence, thus cognizable by the eye, requires 

 to be more particularly explained. Though some of the larger 

 lines in the gaseous spectrum coincide with some of the larger 



3x2 



