170 Mr. H. E. Ives on the 



higher or lower illumination than the earlier curves. It is 

 thus out o£ the question to ascribe a certain luminosity curve 

 to a definite illumination, since the same illumination in 

 metre candles corresponds at different times to different 

 retinal conditions. Where then in the appended curves the 

 illuminations are given for the low illumination observations, 

 the figures indicate under what conditions the measurements 

 were made, but do not necessarily mean that any one curve 

 is only to be obtained at just that number of metre candles. 

 The order of illumination is the important thing here, and 

 the trend of the phenomena with changing illumination. 



Work on the border-line between physiology and physics 

 tends to shake one's faith in the principle of universal causa- 

 tion. Faith may be reconciled with experience only by 

 constant reminder that however exactly physical conditions 

 may be reproduced, one's physiological apparatus of seeing 

 cannot be brought to a previous condition by any amount of 

 careful work with metre stick or voltmeter. 



These remarks on the shift of one's illumination scale 

 apply only to illuminations of the order of twenty units and 

 below ; above that figure this difficulty disappears, and for 

 that reason the reproducibility test described above was made 

 at high illumination. Needless to say, the difficulties of low 

 illumination measurements have necessitated close applica- 

 tion and considerable repetition of work in order to secure 

 the data presented. 



(c) Effect of changing illumination } on each method, 

 for each field- size. 



In Plates I. and I. a (PI. IV.) are given luminosity curves as 

 obtained by the two observers by whom the complete sets of 

 measurements (illumination and field-size varied) were made. 

 These as plotted are corrected for prismatic dispersion, but not 

 for the energy distribution of the source, and are arbitrarily 

 made equal at '574 //,. In the upper row are shown the 

 curves obtained by the flicker method, four different illumi- 

 nations being plotted together, in three groups, each repre- 

 senting one size of field. In the lower row the same data 

 are given for the equality of brightness method. Appended 

 keys indicate the illumination and field sizes in question. 



From these curves it appears that the effect of change of 

 illumination is to shift the luminosity curves along the 

 spectrum. The most striking phenomenon is that while 

 the shift with the equality of brightness method is toward 

 the blue (Purkinje effect), the shift with the flicker method is 



