172 RESPONSE IN THE LIVING AND NON-LIVING 
The length of the tube is so adjusted that the disc is at 
the distance of most distinct vision from the end of the 
tube applied to the eye. The blackened disc is turned 
towards a source of strong light, and a short exposure 
is given by the release of a photographic shutter inter- 
posed between the disc and the eye. On closing the 
eye, immediately after a short exposure, it will at first 
be found that there is hardly any well-defined visual 
sensation; after a short time, however, the writing on 
(6) 
oO” 10” 20)” 30” 40" 
Fic. 110.—RESPONSE-CURVES OF THE SENSITIVE SinvER CELL 
Showing greater persistence of after-effect when the stimulus is strong. 
(a) Short exposure of 2” to light of intensity 1; (b) short exposure of 2” to 
light nine times as strong. 
the blackened disc begins to appear in luminous 
characters, attains a maximum intensity, and then fades 
away. In this case the stimulus is of short duration, 
the light being cut off before the maximum effect is 
attained. The after-effect here is positive, there beimg 
no reversal or interval of darkness between the direct 
image and the after-image, the one being merely the 
continuation of the other. But we shall see, if light is 
cut off after a maximum effect is attained by long 
