156 Mr. H. E. Ives on the 



or critical frequency in their relation to illumination have 

 seriously proposed using them as means of exact photometry. 



The last method to be considered is that of flicker 

 photometry. The flicker photometer owes its applicability 

 to heterochromatic photometry to the fact that colour fusion 

 occurs before brightness fusion ; that is, the sensation of 

 nicker caused by alternation of one colour with another dis- 

 appears at slow speeds of alternation, resulting in a field of 

 uniform hue in which is an outstanding flicker. This out- 

 standing flicker disappears only when the two illuminations 

 bear a certain relation to each other. For lights of identical 

 colour the point of disappearance of flicker corresponds 

 exactly with the point of equal illumination. With lights 

 of different colour this point certainly corresponds closely 

 with equality as measured by other methods, though perhaps 

 not exactly. 



The flicker method has found many advocates because 

 of its superior definiteness as compared with the method 

 of direct comparison. The comparison of different hues is 

 eliminated, apparently by a process which separates luminosity 

 from hue, leaving a phenomenon — flicker — of considerable 

 delicacy. Whitman, Rice, and others find settings between 

 lights of different colours nearly as easy as between lights 

 of the same colour. The accuracy of setting compares well 

 with that of direct comparison between lights of the same 

 colour. 



Work by Whitman and Tufts would appear to show that 

 the nicker method possesses this further requirement of a 

 method of measurement: that several illuminations acting 

 together measure to the arithmetic sum of their values 

 measured alone. In short, the flicker method appears to 

 yield greater definiteness than the method of equality of 

 brightness, and to have the same qualifications of a method 

 of measurement as Abney's work would show for the latter. 

 Spectral luminosity curves obtained by the flicker photometer 

 have shown considerable similarity to those by other methods, 

 so that the special requirements of a method of coloured 

 light photometry appear to be well met. 



From this review it seems that the study of methods of 

 heterochromatic photometry would most profitably centre 

 around the equality of brightness and flicker methods. Con- 

 census of practical opinion agrees with this, for it is with the 

 different values given by these two methods that photo- 

 metrists are now largely concerned. From the manner in 

 which each meets the requirements outlined above, it might 

 seem a« if both are suitable, with the advantage in favour of 



