The Sun 



former, and likewise containing sodium vapour, 

 we shall see the two existing bright lines replaced 

 in the same situations by two dark lines showing 

 on the feebly-illuminated field of the spectro- 

 scope; this is the phenomenon of the inversion 

 of lines. 



All these properties will be of use to us in 

 our study of the solar spectrum. In order to 

 carry out this study an image of the sun, about 

 twenty to twenty-five centimetres in diameter, 

 is produced by means of a suitable combination 

 of lenses, and is examined under the spectro- 

 scope in all its parts. Most frequently, that is to 

 say unless the instrument is directed towards 

 a spot or the extreme rim of the disc, the well- 

 known spectrum of the sun is observed, that 

 which constitutes the greater part of its radiation. 

 It comprises the entire scale of colours from 

 red to violet, without counting its invisible 

 prolongations: the heat -giving or infra-red 

 spectrum and the chemical or ultra-violet 

 spectrum. It is, beyond doubt, the emission 

 spectrum common to all bodies, solid or liquid, 

 when raised to a high temperature. We are 



173 



