﻿On the Visibility of R idiation. 305 



In fig. 3 are included the newdati for the violet and 

 extreme red given in tie following table (p. 306) for five 

 subject-. Th »se data were obtained by means of the mercury 

 lines 406, 436, 492, 546, and 578, together with helium lines 

 439, 447, 492, 5 >2, 586, and 568 checked against the acety- 

 lene spectrum for energy. 



Fig. 3. — Visibility Curves. 



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( o o ). Kdnig's data reduced by Nutting, 1907. 



( ). Ives' mean of 18 subjects. 



( ). Author's mean of 21 subjects. 



( ). Curve calculated by formula 1. 



The theoretical formula who<*e values are given in the 

 table and plotted in fig. 3, is ol the form 



V=V m R a ^- E ) 



(1) 



in which H = X may ./A-, and a = 181. The curve computed for 

 the constants A m = - 555 and a = 181 agrees very well with 

 the data of the new mean experimentally determined curve 

 between wave-lengths *48 and *65 n. The departure from 

 the actual curve in the extreme red and violet is of slight 

 consequence in computing the luminosity of sources on 

 account of the relatively low visibility of radiation in those 



Phil. Ma Q . S. 6. Vol. 29. No. 170. Feb. 1915. 



