ON THE ORIGIN OF THE PLANETARY SYSTEM. 153 



in found in the spectrum of any terrestrial element. 

 Apart from the proof of two well-known terrestrial 

 elements, this discovery was of the utmost importance, 

 since it furnished the first unmistakable proof that the 

 cosmical nebulae are not, for the most part, small heaps 

 of fine stars, but that the greater part of the light 

 which they emit is really due to gaseous bodies. 



The gaseous spectra present a different appearance 

 when the gas is in front of an ignited solid whose 

 temperature is far higher than that of the gas. The 

 observer sees then a continuous spectrum of a solid, 

 but traversed by fine dark lines, which are just visible 

 in the places in which the gas alone, seen in front of 

 a dark background, would show bright lines. The 

 solar spectrum is of this kind, and also that of a great 

 number of fixed stars. The dark lines of the solar 

 spectrum, originally discovered by Wollaston, were 

 first investigated and measured by Fraunhofer, and are 

 hence known as Fraunhofer's lines. 



Far more powerful apparatus was afterwards used 

 by Kirchhoff, and then by Angstrom, to push the de- 

 composition of light as far as possible. Fig. 6 re- 

 presents an apparatus with four prisms, constructed 

 by Steinheil for KirchhofF. At the further end of the 

 telescope B is a screen with a fine slit, represent- 

 ing a fine slice of light, which can be narrowed or 

 widened by the small screw, and by which the light 



