38 MEMORIE DELLA SOCIBTa' 



On tlie Determination of the Wave-Lengths of tlie Ultra-Yiolet Eays, 

 from Photograplis made with a Diffraction Grating. (Tllustrated hy a 



Photograph of the Diffraction Spectrum of the Sun). 



By Henry Draper, M. D. Professor of Physiology 

 and Analytical Chemistry in the University of New York. 



Scientific men bave hitherto depended for the deterniinatiou of the characteristic 

 lines in the spectra of the elements, the Sun, etc, npon observations made on the 

 visible rays, and but little attention has been directed to the well marked lines 

 both above H and below A. Photographs of the by prismatic spectrnm bave been made 

 3, W. Draper, Becquerel, Miiller, Mascart and others, and more particularly in re- 

 cent times by Rutherfurd. These last leave little to be desired in the region from 

 just below F, to just above H, for while they are on a largo scale, they are shar- 

 ply defiued. 



But the study of the ultra-violet , and ultra-red rays, by meaus of a prismatic 

 spectrum, has the disadvantage of not representing the lines in the true order of 

 their wave-lengths , and nioreover the prismatic spectra of various observers are 

 not comparable readily with one another, inasmnch as the irrationality of disper- 

 sion varies with the composition of the prism. It is difficult therefore to deterraine 

 the wave-lengths by such means , and to arrive at the exact resnlts that modem 

 Science demands. 



For these reasons it has seenied to be desirable to produce photographs of the 

 diffraction spectrum originating from a finely ruled gratiug, and to give them as 

 great a leugth and as precise definition as possible. Experiments bave been made 

 formerly in tbis direction by J. W. Draper in 1844, by Mascart in 1866, and by 

 Cornu in 1872, but none of their resnlts are entirely satisfactory. 



The picture which accompanies this memoir is the result of a year's work in tbis 

 direction, and was produced in the foUowing manner. I first took, by means of a 

 diffraction grating, a photograph of the spectrum of the Sun, about 12 inches (0.305 



