

CHAPTER XVI. 



INTRODUCTION OF PHOTOGRAPHY. 



1. Method Employed. 



THE method of photography that was adopted may now 

 be stated. One object was to compare the light of the sun 

 with the light of the vapour, in the electric arc, of any particular 

 substance that we wished to observe. Such a comparison of the 

 Fraunhofer lines, with those visible in the spectrum of the 

 vapour of each of the metallic elements would enable us to 

 study each line in detail and finally to construct a new map 

 of the solar spectrum. 



But it may be said, " Surely if you are going to limit yourselt 

 to photography, you will only be dealing with a very small part 

 of the spectrum." My reply to that is that the remark is 

 perfectly true, but practically the portion thus investigated was 

 a new one, and so it gave us fresh ground on which to base our 

 inquiries. But it was not long before other laboratory work 

 gave us reason to believe that what was then being done in 

 photography at the blue end of the spectrum would be done by 

 photography in every other portion, for in fact a spectroscopic 

 study of the behaviour of bodies at low temperature, to which I 

 shall refer in the sequel, had led several to believe at all events 

 had led me to believe that what one got in the text- books about 

 actinism and so on was but a very rough approximation to the 



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