THE THRESHOLD OF VISION FOE DIFFERENT COLOURED LIGHTS. 
103 
theory, the results indicate that in the case of W. the fovea is free from rods while the 
number of rods increase rapidly as we go from tire fovea. In the case of B., however, 
the distribution of rods, at any rate up to at least 10 degrees, is very nearly uniform, 
if anything there being an excess at the fovea. It is, however, possible that the lower 
value for the threshold obtained at the fovea may be due to the better definition of the 
SSN 
Fig. 6. _ 
image formed at the fovea, so that the falling off of the sensitiveness as we go from 
the fovea may possibly be due to spherical aberration of the eye. 
In fig. 6 the curves corresponding to tire foveal measurements for all the observers are 
plotted together, and it will be seen that in the case of observers W., T., Bk., Pi. and 
N. the curves are in very fair agreement, and we may classify these persons as belong¬ 
ing to a single class, I., who have a fovea practically free from rods. Observers B., A. 
