104 
SIR W. DE W. ABNEY AND PROF. W. WATSON ON 
and Ar. on the other hand belong to another class, II., who have a more or less 
plentiful supply of rods at the fovea. 
Since persons belonging to class II. so far appear to be less common, and as much 
greater individual variation occurs in this class than in class I., we give the series 
of curves for the two other observers of class II. It will be noticed that in the case of 
A (fig. 7) the threshold values at the fovea are the highest except in the red, while the 
values at 1’25 degrees from the fovea are the lowest. The differences for the different- 
parts of the region studied are not however very great, so that up to 10 degrees from 
the fovea the distribution of rods is fairly uniform. In the case of Ar. (fig. 8) there 
is a fairly uniform decrease in the threshold values as we go out from the fovea, at any 
rate up to 10 degrees. 
