vi.] THE COMPARISON PRISM. 73 



prism in front of it, by means of which he could reflect into 

 one-half of it the light from the incandescent vapour of the 

 substance under examination, while the other half received the 

 direct light of the sun. It was quite easy by this method to 

 see in his observing telescope no longer the spectrum of the sun 

 alone, but the spectrum of the sun together with the spectrum 

 produced by the incandescent vapour of each of the chemical 

 substances which he chose to experiment upon. 



While the spectroscope was growing in perfection, great im- 

 provements were also made in the methods of producing 

 incandescent vapours. Especially was this advance important 

 in gradually pressing into service the use of higher and higher 



FIG. 31. Patli of light through comparison prism, d, f, o, prism ; L, light- 

 source ; r, point of reflection ; s, slit ; F, light-sourca in front of slit. 



temperatures. When the earliest spectroscopic observations 

 were made, the highest available temperature was practically as 

 we have seen that afforded by a spirit-lamp. It is true that the 

 very earliest workers, Wollaston and Fraunhofer, both employed 

 electricity in their investigations, but they did not employ it in 

 what we should to-day consider an available form. The inven- 

 tion of the Bunsen burner was the first step in advance. The 

 first extensive employment of electricity in spectroscopic work 

 was by Sir Charles Wheatstone (1835), who examined the 

 spectra of mercury and various other metals obtained in a molten 

 state by passing a rapid succession of electromagnetic sparks 



