586 MANUAL OF PHYSIOLOGY. 



of two lights, the better can we distinguish 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 

 electric 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 considerable 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 terminals as the waves of sound excite the endings of 

 the auditory nerve. But we know that light has a very distinct 

 action on many chemical combinations, such as reducing salts of 

 silver and gold, etc. We, therefore, imagine that the light waves 

 set up, in the outer layer of the retina, certain intermolecular 

 motions or chemical interchanges, the result of which is that the 

 nerve fibres are stimulated to activity and transmit an impulse 

 to the brain. In the outer layer of the retina the light may be 

 said to produce a change in the retina which, in some respects, 

 may be compared to that which occurs on the sensitive photo- 



