584 MANUAL OF PHYSIOLOGY. 



the darker the background the weaker the illumination 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 stimulation, for we find the more 

 the light is intensified the less we are able to notice a fresh incre- 

 ment 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 of two lights the better can we dis- 

 tinguish any difference that may exist between them. 



The effect lasts for a noticeable time after the stimulus has been 

 removed, particularly if the light be very intense. This can well 

 be seen when a brilliant point is observed in rapid motion ; instead 

 of a point a streak of light is seen. Thus falling stars leave a 

 line of light after them caused by the persistence of the stimulus, 

 and a luminous body rapidly rotated gives the impression of a 

 circle of fire and not of a moving point. 



When the stimulus is very intense, such as is caused by an elec- 

 tric light or when we look at a bright object like the globe of a 

 lamp steadily for some time, then the effect persists for a very con- 

 siderable time, and even after the eyes are shut we see a distinct 

 image of the object. This is called the positive after image. If 

 the retina be exposed to a bright light until it be fatigued, and 

 then suddenly turning we gaze at a white wall, the bright part of 

 the positive after image is replaced by a dark figure which is 

 termed the negative after image. 



A strong stimulus applied to the retina spreads from the part 

 upon which the bright image falls to the parts in its immediate 

 neighborhood, so that the bright object looks larger. This phe- 

 nomenon is called irradiation. It helps to explain many of the 

 peculiarities of vision. 



The question now arises, how does the retina, or rather its layer 

 of rods and cones, convert light into a nerve stimulus? It would 

 appear quite out of the question that the 456 to 700 billions of 

 waves of light per second could mechanically excite the nerve 



