L883.] Prof. G. D. Liveing on the Ullra-Violet Spectra. 245 



WEEKLY EVENING MEETING, 



Friday, March 9, 1883. 



Sir Frederick Pollock, Bart. M.A. Vice-President, in the Chair. 



Professor George D. Liveing, M.A. F.K.S. 



The Ultra-Violet Spectra of the Elements. 



[t seems probable that the range of our vision as regards colour is 

 jlosely connected with the intensity of that part of the solar radiation 

 .s^hich reaches us on the earth, for Langley's observations on the in- 

 ;ensity of the sun's rays in different parts of the spectrum bring out 

 ihe fact that the region of greatest intensity falls nearly in the middle 

 )f the visible spectrum, and includes those colours to which our eyes 

 ire most sensitive. The ultra-violet rays, those which lie beyond the 

 ;^iolet on the more refrangible side, are not, however, absolutely 

 nvisible, for, by carefully excluding light of lower refrangibility, 

 Jerschel found that he could see some distance beyond the Fraun- 

 lofer line H, into what he called the lavender-grey ; and Helmholtz 

 las succeeded in seeing nearly all the strong lines in the solar spec- 

 Tum almost or quite up to its limit. Still these rays may fairly be 

 iaid to be beyond ordinary vision ; and from their power of chemical 

 iction they used to be distinguished as " actinic " rays. We know 

 ,; low that they have no monopoly of chemical activity, and we recognise 

 10 difference between luminous and actinic rays, the visible and the 

 iltra-violet, except in their oscillation frequencies ; that is, in the 

 ate at which the successive pulsations of the ray succeed one another, 

 ■nd in the colour and refrangibility which are directly dependent on 

 hat rate. That the ultra-violet part of the solar spectrum extended 

 .t least as far above the line H as F is below it, has been known 

 ince the time of Wollaston, who observed its effect in blackening 

 ilver salts ; but it is only about twenty years since Stokes made 

 :nown to us the great length and intensity of the ultra-violet 

 pectrum of the electric spark. Stokes used his own invention, a 

 luorcscent screen, for observing the rays ; and at the very time 

 vhen Stokes published his discovery, W. A. Miller published photo- 

 graphs of the spectra of sparks taken between various metallic 

 ilectrodes. Both these methods, that of fluorescent screens and that 

 . "f photography, have been used by Professor Dewar and me in our 

 esearches. For the method of fluorescence we have used a modifica- 

 iion of Soret's eye-piece, substituting for the uraniiun glass-plate a 

 redge-shaped vessel full of a solution of sesculine, placed with 

 ts edge horizontal so that we look down on the fluorescent liquid. 

 ?he wedge form of the vessel has the advantage of refracting out of 

 he line of vision all the rays except those which produce fluorescence, 



