THE THRESHOLD OF VISION FOR DIFFERENT COLOURED LIGHTS. 
109 
The threshold in the red, we are assuming, depends on the cones, and N.’s cone vision 
is defective since he has a shortened spectrum, hence his high threshold values 
at the red end. 
Piper (‘ Zeit. f. Psychol, u. Physiol, d. Sinn.,’ 32, 161, 1904; see also Helmholtz 
‘ Physiol. Optik,’ 3rd edition, vol. 2, 286) found that with the dark adapted eye, and 
with stimuli which approach the threshold, binocular summation occurs, so that when 
a feebly illuminated surface was examined with both eyes it appeared nearly twice as 
bright as when one eye only was used. He also found that the threshold values for 
the two eyes used together were about half the values for either eye used separately. 
It seemed of interest to examine whether this summation efiect would be the 
same for observers of the two classes, and hence W. (class I.) and B. (class II.) made 
Q 2 
