CONDITIONS AFFECTING VISION. 585 



The stimulus need only be applied for a very short time in 

 order to cause a distinct sensation, for we can distinctly see a 

 single electric spark ; it need only be applied to an extremely 

 small part of the retina, as we can, by direct vision, see a very 

 minute speck of light, and a very feeble ray suffices to stimulate 

 the retina. The amount of stimulation produced depends upon 

 (1) the intensity of the light, i. e., the amount of light received 

 in a given area ; (2) the duration of its application ; and (3) the 

 extent of retina to which it is applied ; (4) the part of the retina 

 stimulated ; (5) the darker the background the weaker the illu- 



FIG. 230. 



Section of the retina at the yellow spot, showing the great number of cones (a) at this 

 point, and the thinness of the other layer. (Cadiat.) 



mination we can distinguish, i. e., the greater the stimulating 

 effect of a weak light ; (6) by fatigue the retina loses its power 

 of appreciating light, and more stimulus is required to produce 

 a given effect. On waking, the daylight is at first dazzling, but 

 soon the retina can bear the stimulus. An increase of intensity 

 of light does not cause an exactly proportional increase of stimu- 

 lation, for we find the more the light is intensified, the less we 

 are able to notice a fresh increment of light until a degree of 

 intensity is arrived at, when no further addition can be detected, 

 and the light becomes blinding. The less the absolute intensity 



