Effect of the Ultra-violet Spectrum. 299 1 



neighbourhood of w=l'65 a comparatively large change of 

 refractive index involves only a small change in wave-length, 

 and therefore the wave-lengths used at n = l*65 differ only 

 slightly from those at ?i = l*66. With regard to the minimum 

 a rather closer study of the region of this point was made 

 by taking readings for the ranges of spectrum with centres 

 at n = 1*665 and ?i = 1*675 ; both of these readings were 

 larger than for /z = l'67 showing that the actual minimum 

 cannot be very far from ra=l'67. The wave-length corre- 

 sponding to ?i = l*67 is not known, Rubens' readings only 

 going as far as n = l*65. 



An absorption band is, however, known to exist in this 

 region, and quartz is supposed to cut off all rays of wave- 

 length less than 185 [ifi. Wood (Physical Optics, p. 324) 

 calculates the ultra-violet absorption band at 103 jx/i. It 

 seems likely that the electrical minimum at n=l*67 is 

 coincident with the absorption band in quartz, in which case 

 the statement that quartz is opaque to rays less than 185 /xyu. 

 in wave-length is erroneous, for the second maximum which 

 was always well marked is beyond this limit. In any case it 

 is highly improbable that for ;< = 1*70, at which the electrical 

 effect was measurable, the wave-length should not be less than 

 185 /jl/jl, for it is unlikely that from »=1'65 to n = l*70 the 

 wave-length should not change by more than 15 /jl/jl. 



It is interesting to note that in Pfluger's research on the 

 energy of the ultra-violet spectrum, conducted under very 

 much the same conditions as the present electrical one, except 

 that he used a fluorite prism, the energy commencing in the 

 visible spectrum was very small until he reached \ = W0 fi/x 

 when it rose to a very strongly marked maximum. This 

 would coincide very nearly with the first maximum in the 

 electrical curve. Too much stress should not, however, be 

 laid upon this, as for reasons indicated previously there is not 

 to be expected any simple relation between the energy of the 

 spectrum and the electrical effect. It is probable that the 

 electrical action is of the nature of a resonance effect in which 

 case the energy would not be the primary factor though it 

 would of course contribute. Except at the maximum there 

 is no similarity between the energy curve of Pfliiger and 

 fig. 4. A final point of interest is that hitherto it was 

 supposed that there was no electrical action in the visible 

 spectrum, but with the sensitive methods of measurement 

 used in this research a small effect was detected in the visible 

 spectrum (which extended from n = 1*54 to n = 1*56) though it 

 was only two per cent, of the maximum effect. To make 

 certain of the presence of this, a plate of glass was inserted 



