160 RESPONSE [IN THE LIVING AND NON-LIVING 
increase of duration does not increase magnitude of 
effect. Too long an exposure may however, owing to 
fatigue, produce an actual decline. 
I give here two sets of curves (fig. 102) illustrating 
the effect of lengthening exposure. The intensities of 
light in the two cases are as | to 4. The incandescent 
burner was in the two cases at distances 50 and 25 em. 
respectively. It will be observed that beyond eight 
seconds’ exposure the responses are approximately 
uniform. Another noticeable fact is that with long 
exposure there is an after-oscillation, This growing 
effect with lengthening exposure and attainment of 
limit is exactly paralleled by responses of retina under 
similar conditions. 
Relation between intensity of light and magnitude of 
response.—In the responses of retina, it is found that 
increasing intensity of light produces an increasing 
effect. But the rate of increase is not uniform: increase 
of effect does not keep pace with increase of stimulus. 
Thus a curve giving the relation between stimulus and 
response is concave to the axis which represents the 
stimulus. 
The same is true of the sensation of light. That is 
to say, within wide limits, intensity of sensation does 
not increase so rapidly as stimulus. 
This particular relation between stimulus and effect 
is also exhibited in a remarkable manner by the sensi- 
tive cell. For a constant source of lght I used an 
incandescent burner, and graduated the intensity of the 
incident light by varying its distance from the sensitive 
cell. The intensity of light incident on the cell, when 
