IV. On a new method of spectrum observation

Abstract

In anticipation of my report on the Methods of Mapping Spectra, which I have been requested to prepare for the Solar Committee, I beg to present to them the following account of some recent work which has been suggested during the preparation of that report. In the "Phil. Trans, for 1873 I gave an historical account, showing how, when a light source, such as a spark or an electric arc, is made to throw its image on the slit of a spectroscope, the lines had been seen of different lengths, and I also showed, by means of photographs, how very definite these phenomena were. It was afterwards demonstrated that chemical combination or mechanical mixture gradually reduced the spectrum by subtracting the shortest lines, and leaving only the long ones. On the hypothesis that the elements were truly elementary, the explanation generally given and accepted was that the short lines were produced by a more complex vibration imparted to the “atom” in the region of greatest electrical excitement, and that these, vibrations were obliterated or prevented from arising by cooling or admixture with dissimilar atoms.