108 
SIR W. DE W. ABNEY AND PROF. W. WATSON ON 
obtained by observers of class II. While the threshold values for observers A. and B. 
are definitely lower than any of the others, observer Ar. has at 1'25 degrees threshold 
values higher than those of the majority of class 1. ' 
In figs. 10, 11, and 12 are collected the threshold curves for 2'5, 5, and 10 degrees, 
and it will be seen that in each case there is no distinction between the two classes. 
With regard to the observers having abnormal colour vision, Ar. and It. have 
threshold values which do not differ materially from the values obtained by persons 
with normal colour vision. The red blind observer N. has a markedly higher threshold 
value at the red end of the spectrum, in fact the spectrum employed in the measure¬ 
ments was not sufficiently bright for him to obtain readings beyond SSN 55 (625 /ufx). 
Id the green, blue, and violet, his values for the threshold agree with the normal. 
