310 Messrs. H. E. Ives and E. F. Kingsbury on the 



impressions as transmitted to the receiving apparatus. Each 

 is due to the same illumination (by different colours) acting' 

 on complementary disks. The critical speed is nearly the 

 same for each, consequently the fractional range of each is 

 nearly the same. This, however, means that the actual 

 range is much less for the disk of small opening than for 

 the disk of large opening. The summation of the two 

 impressions is not therefore constant, but partakes more of 

 the nature of the impression due to the larger opening. In 

 order to make the summation constant it is obvious that the 

 illumination of the smaller disk must be increased, whereby 

 the range of fluctuation of the impression due to it will be 

 made larger, and at the same time the diffusivity proper to 

 its colour will be increased, which will assist to the same end. 



It follows from this reasoning that that colour of' two under 

 comparison ivhich is exposed for the shorter time will he under- 

 rated. 



An important point to notice is that this condition holds 

 only if each colour under comparison preserves its indi- 

 viduality to some extent in the process of transmission, as 

 it would do if its transmission is due to a separate mechanism 

 (e. g. set of nerves) . It is only by this means that the two 

 complementary impressions can be assigned different diffu- 

 sivities and consequently different ranges. In the case of 

 no colour difference the two impressions must be transmitted 

 by the same channel and the diffusivity must be the same 

 for both — that due to the mean illumination. When the 

 illumination is the same on both sides of such an unsym- 

 metrical flicker photometer the two impressions will exactly 

 dovetail, with a constant summation, which is obviously the 

 case. 



This point has bearing on the question of the exact com- 

 putation of the flicker photometer's behaviour with different 

 colours. We cannot assume that each separately distinguish- 

 able colour has its own channel. Three-colour vision theory 

 would point to three such channels as probable. We must 

 therefore expect that the diffusivity governing the trans- 

 mission of each colour is, to a greater or less extent, 

 modified by the other colour under comparison. This means 

 that such computation as can be made on the basis of each 

 colour having its own diffusivity can be expected to be only 

 qualitative. In the previous paper we have confined our- 

 selves to such qualitative study of the effects of different 

 colours, and a similar restriction holds with the following 

 discussion of the effects of unequal exposures with different 

 coloured lights. 



