CHAPTER I. 



INTRODUCTION. 



The following experimental investigation was performed in the 

 Physical Laboratory of Cornell University during the academic year 

 1904-5. It forms the second part of an investigation of infra-red radia- 

 tion, rendered possible by a grant from the Carnegie Institution of Wash- 

 ington. The first grant was for "Investigating infra-red emission and 

 absorption spectra." Finding it impossible to complete the work in the 

 time allotted, the Institution very generously renewed the grant, and 

 the writer takes this opportunity to express his gratitude for the assist- 

 ance rendered. In the Physical Laboratory he is under deep obligations 

 to Profs. E. L. Nichols and E. Merritt for advice and criticisms as well 

 as for the numerous facilities placed at his disposal. His dealings with 

 the two institutions have been so agreeable that it is with regret that 

 additional phases of the work could not be continued with them. 



HISTORICAL. 



i 

 Infra-red Spectrum. 



In the visible and ultra-violet regions of the spectrum the most suc- 

 cessful method of mapping the position of emission lines is by means 

 of photography ; and so long as we do not desire a measure of the en- 

 ergy in these lines this method is very satisfactory. On the other hand, 

 in the infra-red, the photographic plate has never been made sensitive 

 to rays of w'ave-length greater than about 1.2 /x. Other methods have 

 been resorted to, one of the earliest being a phosphorescent plate. This 

 method consists in placing the phosphorescent material (which is made 

 into paint) upon an even surface and exposing it to the spectrum. The 

 infra-red lines have the property of extinguishing the phosphorescence, 

 thus leaving dark lines upon a bright background. The plate thus ex- 

 posed is now placed in contact with a photographic plate which will be 

 acted upon by the phosphorescing parts, but not by the darker lines. 

 The photographic plate when developed is the equivalent of an ordi- 

 nary positive plate. Becquerer was one of the first to apply this method 

 with success. He photographed quite a number of lines of the alkali 

 metals in the carbon arc to 1.2 fi. Recently this method was taken up 

 anew by Lehmann," who was able to extend his investigations to i.y fi. 



Phosphorescent substances sensitive to radiations beyond I.//* are 



^E. Becquerel : Compt. Rend., 99, 374, 1884. 



^Lehmann: Phys. Zeit., 5, 823, 1904. See also Sci. Abstracts, 57, 425, and 

 (1903) ; 750 (1905) ; Ann. der Physik (4), 5, 633, 1901. 



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