176 Mr. H. E. Ives on the 



Plate iv. (PL V.) exhibits the effects of changing illumina- 

 tion for the two methods, for each observer, with the mean 

 curve of all. The chief phenomena of the previous measure- 

 ments are here borne out. With the flicker method, decreased 

 illumination increases the red luminosity ; with the equality 

 of brightness method the same change increases the blue 

 luminosity (observer M.L. at this illumination shows merely a 

 broadening of the curve). In the case of the less experienced 

 observers the equality of brightness measurements are more 

 or less wild, the flicker measurements very good. 



Plate v. exhibits the relative positions of the two kinds of 

 curves for each observer, with the mean. At the low illumi- 

 nation the equality of brightness curves are all higher on the 

 blue side than are the flicker. At the high illumination two 

 observers place the flicker maximum on the red side, two on 

 the blue side of the equality of brightness maximum ; one 

 observer places the maxima about together, although, as has 

 before been noted, the curves do not actually coincide, being 

 of different areas. 



The mean high illumination curves obtained by the two 

 methods show close agreement in the position of the maxima ; 

 and it is not unreasonable to expect that with more observers 

 or more observations extended over such an interval that the 

 observers would not remember their previous equality of 

 brightness criteria, the two mean curves would show practical 

 agreement as to shape and position of maxima. On the other 

 hand, it is apparent that an exact coincidence of the curves 

 may not confidently be expected, but rather two curves of 

 the same shape of slightly different areas. 



Figs. 7 , 8, 9, and 10 (PL III.) give the luminosity curves 

 of all observers plotted together. It is obvious from these 

 that even with workers of normal colour vision considerable 

 differences may be expected (of the order of five to fifteen per 

 cent.) in their measurement of pure colours against an un- 

 saturated colour such as the light of an incandescent lamp. 

 The observers maintain substantially the same relative posi- 

 tions with respect to each other, so that the different positions 

 of their luminosity curves probably represent real differences 

 in colour vision. The flicker curves lie closer together than 

 the equality of brightness, so that in general the observers 

 will differ less on their flicker measurements than on the 

 others. 



It is clear, however, from these curves that whichever 

 method of measurement is employed, we cannot expect to 

 obtain results in exact agreement from observers taken at 

 random, where lights of different colour are to be compared. 



