INTRODUCTION. 



By R. W. Wood. 



In spite of the ven' large amount of work which has been done on ab- 

 sorption spectra, there exists practically no collection of photographed spectra 

 from which one can pick out the media most suitable for any particular line 

 of investigation. The greater part of the published records are drawings 

 made from visual observations, and give no information regarding the optical 

 properties of the media in the ultra-violet. It seems desirable therefore to 

 compile a set of photographic records which are free from the errors liable 

 to enter into observations made by visual methods, and to arrange them in 

 such a way that the medium or media necessary to secure a desired result 

 could be readily found from a mere inspection of the plates. 



A great deal of experimental work was necessarj^ before satisfactory^ 

 photographic records were obtained. The details of the spectrograph and 

 the refinements of the method have been worked out very skilfully by 

 Dr. Uhler, who has done practically all of the experimental work. It was 

 our original plan to include the colored salts of metals, and to examine a 

 large number of colorless substances for peculiarities in the ultra-violet. The 

 solutions of the inorganic compounds could not, however, be investigated 

 in precisel}' the same manner, owing to their less powerful absorption. Much 

 thicker absorbing wedges were required, and these gave trouble, even when 

 compensated, as a result of dispersion. It was therefore decided to limit 

 the present work chiefly to a study of the aniline dyes, which are used to a 

 much greater extent than the metallic salts, in the preparation of absorbing 

 screens. The absorption spectra of a number of metallic salts, however, 

 have been photographed, as it is believed that many of them will be useful 

 in the preparation of ray filters ; some of them are far more transparent in 

 the ultra-violet than the aniline dyes. Even such substances as the salts 

 of erbium, neodymium, and praseodymium are useful in special cases where 

 it is desired to suppress one or more isolated spectral lines. For example, 

 a solution of neodymium has a very narrow and intense band coincident with 

 the D lines, and has therefore the property of cutting out the sodium radia- 

 tion from a given source, transmitting at the same time nearly the whole of 

 the remainder of the spectrum. The same salt can be used to advantage 

 when working with the new cadmium and zinc arc lamps in quartz tubes, 



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