Effect of the Ultra-violet Spectrum. 



297 



from the following example ; the ordinate at the point 

 n = 1*64 represents in some arbitrary units the number of ions 

 given off in 10 seconds by the zinc plate when light having 



Fi<r. 4. 



Q 

















1 



























J 











/ 



r\ 













/ 







/?£ Fff/IC T/l/E \//VDEX. 



Spectrum 



refractive indices between 1*035 and 1*645 fell upon it. The 

 units are of necessity arbitrary because the actual number of 

 ions given off is unknown. The limitations of the curve may 

 be briefly recapitulated here. In the first place it simply 

 refers to the spectrum produced by a quartz prism, no 

 correction having been made either for the absorption of the 

 quartz or of the air through which the rays passed, the 

 absorption in both cases increasing with n. Secondly, in 

 order to produce a measurable effect a comparatively large 

 portion of the spectrum is included in each reading, and the 

 curve gives no information about the effect of lines or narrow 

 bands in the spectrum. 



Thirdly it should be noticed that the curve gives the 

 connexion between the number of ions set free and the 

 refractive index. A curve giving the connexion between 

 the number of ions liberated and the wave-length would be 



very different. Rubens' curve shows that •_■-- for the visible 



J da 



spectrum is very big, decreasing very rapidly as n increases, 

 <md if therefore a curve connecting n and \ were drawn it 

 would show that the effects of the small wave-lengths were 

 very much more marked than even the effect of large values 

 of n in fig. 4. This curve has not, however, been drawn 



