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 1 to 4. The incandescent 

 burner was in the two cases at distances 50 and 25 cm. 

 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 light 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 



