The Sun 



separated by dark intervals, occupy in the field 

 of vision of the spectroscope well-defined situa- 

 tions. This property, particularly distinct in 

 the case of hydrogen and of vapours of metals, 

 has enabled each element to be char- 

 acterised by the lines which it produces at 

 a high temperature. Sodium, for example, 

 gives two yellow lines very close together; 

 hydrogen gives four principal lines, one red line 

 very bright, one blue and two others violet ; iron 

 vapour gives an admirable spectrum, consisting 

 of hundreds of lines. 



This is not all; it is possible to identify 

 bodies not only by the light which they emit, 

 but also by the light which they stop, which 

 they absorb. A red glass appears red to us, 

 because it is opaque for all light except red. If 

 a transparent but coloured body is interposed 

 between a source of white light and a spectro- 

 scope, the continuous spectrum produced by 

 the source will appear crossed by a series of 

 broad dark bands or narrow black lines according 

 to circumstances. The bands and lines corre- 

 spond to the absent radiations absorbed by the 



intervening body, and the nature of the body 



171 



