Plucker on Spectrum Analysis. 255 



PLUCKER ON SPECTRUM ANALYSIS. 



The following is a translation of two articles which have 

 appeared in Cosmos, from the pen of the celebrated philosopher 

 of Bonn, and will be regarded as a very valuable contribution 

 to our knowledge of a new and interesting subject. It tends, 

 on the one hand, to correct exaggerated notions of the facility 

 which the new process affords of ascertaining the constitution 

 of the sun, or other remote bodies, while, on the other, it opens 

 a wide field for further research and discovery. M. Plucker 

 observes : — " Spectral analysis, as conceived by me in 1858 — 9, 

 consists in introducing the gas to be examined in tubes, of 

 which one portion is capillary. After having conveniently 

 rarefied the gas by means of a mercurial evacuator, the dis- 

 charge of an induction apparatus is made to pass through it. 

 The electric current, condensing itself in the capillary tube, 

 renders incandescent the gas which it contains. The light is 

 sufficiently bright to afford a beautiful spectrum, which is 

 usually composed of a certain number of brilliant and charac- 

 teristic lines, one of which, whose position is exactly determined, 

 indicates the nature of the gas which is the subject of the 

 experiment. 



" I have thus operated on the ordinary gases and on certain 

 vapours. When the vapour of a substance introduced into the 

 tube has not the density necessary to cause the current to pass 

 through it, a lamp is employed to increase the vaporization until 

 the current traverses it, and produces incandescence. In this 

 way I have treated mercury. Following the same principle, to 

 obtain the spectrum of metallic sodium, I first fill the tube 

 (which I have named after Geissler, the ingenious artist by 

 whom it was constructed) with a neutral gas, hydrogen, whose 

 spectrum is known. 



te The spectra of different bodies in a gaseous state may be 

 divided in several classes, each exhibiting peculiar characteris- 

 tics, and the following considerations arise from the varied 

 appearances they present. If the light received by the spec- 

 troscope contains all the colours whose refrangibility increases 

 from the red to the extreme violet, the continuous spectrum 

 that is obtained is composed of an infinite number of super- 

 imposed bands, of which each has the breadth of the slit as 

 seen through the telescope. The Drummond light offers an 

 example of this kind. If, on the contrary, the incident light 

 only contains a limited number of colours, the spectrum is dis- 

 continuous, the luminous bands being separated by black spaces. 

 These bands tend to become mere lines if the aperture of the 

 slit is reduced. Hydrogen gas and chlorine, together with the 

 VOL. II. — NO. IV. u 



