THE THRESHOLD OF VISION FOR DIFFERENT COLOURED LIGHTS. 
117 
class II., fig. 19. In the case of R. and W. there is a marked rise at the violet end 
of the spectrum. Of the observers of class II., A. and B. have nearly horizontal 
curves throughout the greater part of the spectrum, while Ar.’s curve rises 
markedly towards the violet end, here again resembling the values obtained by 
class I. 
•020 
30 40 
SSJY 
■LU-U-iJ 
50 60 
Fig. 19. 
At 2'5 degrees, 5 degrees, and 10 degrees from the fovea, as shown in figs. 20, 21, 
and 22, the results obtained by the two classes resemble one another, and, except at 
the red end of the spectrum, the product of the extinction into the rod luminosity 
is nearly constant. There is in the case of some observers a tendency for the product 
to increase towards the violet, but it must be remembered that the luminosity 
R 2 
