THE THRESHOLD OF VISION FOR DIFFERENT COLOURED LIGHTS. 
Ill 
readings for the left eye are decidedly lower (a lower annulus reading corresponds to 
a higher threshold value) than for the right eye. Further, that with the two eyes 
together the threshold values agree with those corresponding to the better of the two 
eyes taken alone. At the fovea the threshold values are the same whether both eyes 
are used or either eye separately. In the case of B., figs. 14, 15, the values obtained 
with each eye separately agree with the values obtained with both eyes used 
together. These experiments are at variance with those of Piper, and show that for 
a typical observer of either class and for an object subtending an angle of 34 minutes 
and using momentary stimuli there is no evidence of binocular summation. Whether 
the difference can be accounted for by retinal fatigue, where as in Piper’s experi¬ 
ments the light was kept on continuously, must be left for further experimental 
investigation. 
Another point which has been examined is the question as to the influence of the size 
of the stimulus light on the threshold values. In Table III. and figs. 16 a and 16 b are 
given the.results obtained by W. using discs of diameters 2'8, 6'33 and 13'0 mm. which 
subtended at the eye angles of 14 and 31 minutes and 1 degree 2 minutes respectively. 
Observations were made both at the fovea and at 5 degrees from the fovea. When 
comparing these results with those given in Table II. it must be observed that owing 
to loss of accommodation during the interval which elapsed between the two sets of 
observations W. had to use spectacles when making these latter measurements. In 
the figure the logarithms of the energy at the threshold have been plotted against 
the logarithm of the diameter of the disc on which the stimulus light fell. 
