xvi. J A CASE IN POINT. '227 



These examples serve to illustrate the manner in which large 

 numbers of the coincidences recorded by former observers were 

 disposed of. 



By eliminating lines due to impurities in the manner just 

 described, a spectrum is at length obtained, of which every line 

 is assignable to the particular element photographed, the same 

 temperature being employed in the case of all the elements 

 observed. This is an important point. 



As the mapping progressed, there seemed a probability that 

 the spectra of iron and other metals, which all observers have 

 found very complex, would become much simplified, owing 

 to the elimination of many lines hitherto attributed to these 

 metals being proved in this way to be due to the presence of 

 impurities in them. 



It may not be uninteresting to detail a few considerations 

 which induced me to arrive at such a conclusion. Instances 

 occurred in which a well-defined line appearing in the spectrum 

 of a metal has proved coincident with the longest line in the 

 spectrum of an element newly mapped.- 



Then, again, suppose iron to be present as an impurity in an 

 element which is being mapped for the first time, the longest 

 lines of iron are first looked for, but it may happen that one of 

 these lines is represented in the spectrum of the new element as 

 very much longer than the other lines in the spectrum of iron, 

 which have hitherto been regarded as of about the same length. 



Lastly, there were many lines in the spectrum of an element 

 reversed in the solar spectrum, which were coincident with lines 

 of the same wave-length in the spectra of other elements. It 

 might be that greater dispersion would in all these instances 

 prove that these lines instead of being absolutely coincident, 

 might slightly graze one another ; but my experience, thus early, 

 led me to suspect that these lines were rather due to the pre- 

 sence of common impurities, either in the shape of unmapped 

 elements or of elements hitherto unknown. 



Q 2 



