Absorption Spectra. 223 



Absorption Spectra, and a Method for their more 

 Accurate Determination. By Alex. Hodgkinson, 

 M.B., B.Sc. 



{Received March 18th, 1890.) 



When the various elementary bodies are raised to a 

 sufficiently high temperature they glow with a light having 

 qualities of such a special nature as readily to allow of the 

 identification of such bodies from the nature of the emitted 

 light. Examined by means of the spectroscope such light 

 appears as the so-called radiation spectrum, which consists 

 of the various rays of light constituting the original beam 

 spread out into a series of bright lines, or bands, each of 

 which occupies a constant and definite position in such 

 spectrum in accordance with its special refrangibility. 



Such is the uniformity in the constitution of the light 

 emitted by the same substance, and such the identity in 

 position which rays of the same refrangibility or wave 

 frequency occupy in the spectrum, that this method of 

 analysis, as applied to radiation spectra, is universally 

 recognised as one of the most delicate methods of physical 

 investigation. True it is that, with certain variations as 

 regards temperature, pressure, and motion of the incan- 

 descent body, the position, number, and character of the 

 spectral lines become materially modified ; but these con- 

 ditions are of an extreme and conspicuous nature, and, 

 therefore, are unlikely to escape recognition or cause 

 confusion whenever they may be present. 



If half a dozen careful investigators were asked to supply 

 wave length radiation spectra of any of the elementary 

 bodies, even without specifying any conditions under which 

 O 



