ASTRO-PHYSICS 263 



problems. The spectroscope itself illustrates the 

 progressive triumph of modern science, for it is 

 the work neither of one man nor of one century. 

 Its principles have been developed gradually and 

 its construction elaborated throughout a couple 

 of hundred years. Newton was the first to 

 analyse the light of the sun by a prism, to study 

 the spectrum thus obtained, and to show that it 

 consists of rays of every colour, which, when 

 blended together in the eye, produce the sensa- 

 tion of white light. In the year 1802, Wollaston 

 noticed that the spectrum of the sun's light was 

 crossed by a number of fine dark lines, and, 

 shortly afterwards, the relative positions of these 

 lines were mapped carefully by Fraunhofer, 

 whose name the lines have borne since that 

 time. 



The next great advance was made by the 

 chemists Bunsen and Kirchhoff, who repeated 

 and amplified, in the year i860, an almost for- 

 gotten experiment of Foucault, though the 

 principles which underlie their discovery had 

 previously been understood by Sir George 

 Stokes. Any vibrating system — a child's swing, 

 for example — is set into violent oscillation if 

 impulses are given to it exactly timed to coincide 

 with its own proper period of vibration. Just as 

 the particular piano wires which are tuned to a 

 particular note will be set in vibration when that 

 note is sounded in their neighbourhood, so the 

 molecules or atoms of a gas will be set in 

 vibration by waves of light which possess a 

 period of oscillation corresponding with their 

 own. A complex wave of light, then, passing 

 through a collection of such molecules or atoms, 



