﻿Manchester Memoirs, Vol xlix. (1904), No. IV 



III. On the Determination of Wave-lengths in the 

 extreme Ultra-violet Portion of the Spectrum. 



By H. MORRlS-AlREY, M.Sc. 



Received and read November 29th, 1904. 



The development of the study of the ultra-violet 

 portion of the spectrum was in its earliest stages closely 

 connected with the progress of photography and the 

 discovery of fluorescence. 



The first important contribution to the subject was 

 made by Edmund Becquerel in 1842, in a work containing 

 observations of the effect of sunlight on the recently 

 discovered silver chloride plates. This publication con- 

 tained the first account of systematic observations of 

 spectra in the region beyond the visible violet. The prism 

 and lenses of the spectroscope which Becquerel used were, 

 however, constructed of glass, so that the shortest wave- 

 length which he was able to photograph could not have 

 been less than 3,100 A, the absorption of glass being 

 already very strong at this region. 



This property of glass seems to have been first recog- 

 nised by Stokes, who, in the period 1852 to 1862, used 

 quartz for the optical parts of his spectroscopes, with the 

 result that he was able to detect radiations far beyond the 

 Fraunhofer line P in the spectrum of sunlight. 



The spectral lines were observed by allowing the 

 spectrum to fall on a screen made of some fluorescent 

 material placed in the eyepiece of the telescope. 



The experiments of Piazzi Smyth on the increased 

 range of the spectrum of sunlight when photographed at 

 high altitudes showed that the atmosphere absorbed and 

 kept back the ultra-violet light to a marked degree. To 



December 16, igoJf. 



