362 The Principles of Spectrum Analysis. 



discern it to be better prepared for variety of vision than is the 

 case with the larger part of bony fishes. The crystalline lens 

 is large^ by which means it is able to take in, a wide range of 

 vision, while its situation far back in the chamber, and very 

 near the retina or expanded fibres of the nerve of sight, from 

 which, by bringing the rays of light to a short focus, the dis- 

 tance at which objects would be seen must be small, is changed, 

 and a larger extent of perception secured by the compressing 

 operation of the external muscles of the eye-ball, the lens itself 

 being thus driven forward towards the front. 



THE PEINCIPLES OF SPECTEUM ANALYSIS. 



BY THOMAS EOWNEY. 



Sixty years or more ago Dr. Wollaston detected in the spec- 

 trum obtained from solar light a series of dark bands crossing 

 it throughout its entire length. These lines may be easily 

 seen through a prismatic telescope, of which Mr. Orookes has 

 contrived a simple form. The discoverer does not appear to 

 have thought much of the fact, and seems to have discon- 

 tinued his experiments, as we have no further account of his 

 researches in that direction. It was not until Fraunhofer of 

 Munich announced his independent discovery of the same 

 lines, and showed that they were constant both in number and 

 position, and mapped them out to the extent of more than 

 600, with the most sedulous care, that they came to be regarded 

 as features worth notice. He showed that these lines, which 

 now bear his name, might be found in all spectra, by viewing 

 them through a telescope, whether the source of light were the 

 sun, moon, fixed stars, or planets. He also found them in the 

 electric spark, and in flames coloured by the combustion of 

 metals. These two philosophers might justly lay claim to the 

 honour of having laid the foundation of what is now termed 

 spectrum chemistry. In the words of Dr. Miller,* 



" The inquiry thus launched by Fraunhofer has been followed in 

 four principal branches of research, which niay be described as re- 

 lating to, — 



" 1. Cosmical lines, or the black lines produced in the light of 

 the sun, the planetary bodies, and the fixed stars. 



"2. Blade lines produced by absorption, a class of phenomena dis- 

 covered by Sir D. Brewster; in his observations upon the red vapours 

 of nitrous acid. 



" 3. Bright lines produced by the electric sparh, when taken between 

 different conductors. 



* Lecture reported in Chemical flews, Ko. 123. 



