CH. LVI.] FUNCTIONS OF THE RETINA 791 



jected before the eye.* This remarkable appearance is due to 

 shadows of the retinal vessels cast by the candle ; and it is only 

 when they are thrown upon the retina in an unusual slanting 

 direction that they are perceived. The branches of these vessels are 

 distributed in the nerve-fibre and ganglionic layers ; and since the 

 light of the candle falls on > the retinal vessels from in front, the 

 shadow is cast behind them, and hence those elements of the retina 

 which perceive the shadows must also lie behind the vessels. Here, 

 then, we have a clear proof that the light-perceiving elements are 

 not the inner, but one of the external layers of the retina ; further 

 than this, calculation has shown it is the layer of rods and cones. 

 The data for such a calculation are the dimensions of the eyeball, 

 the distance of the screen from the eye, the angle through which the 

 candle is moved, and the displacement of the figure seen. 



Duration of Visual Sensations. The duration of the sensation 

 produced by a luminous impression on the retina is always greater 

 than that of the impression which produces it. However brief the 

 luminous impression, the effect on the retina always lasts for about 

 one-eighth of a second. Thus, supposing an object in motion, say a 

 horse, to be revealed on a dark night by a flash of lightning. The 

 object would be seen apparently for an eighth of a second, but it 

 would not appear in motion ; because, although the image remained 

 on the retina for this time, it was really revealed for such an 

 extremely short period (a flash of lightning lasting only a millionth 

 of a second) that no appreciable movement on the part of the object 

 could have taken place in the period during which it was revealed to 

 the retina of the observer. The same fact is proved in a reverse 

 way. The spokes of a rapidly revolving wheel are not seen as 

 distinct objects, because at every point of the field of vision over 

 which the revolving spokes pass, a given impression has not faded 

 before another replaces it. Thus every part of the interior of the 

 wheel appears occupied. 



The stimuli which excite the retina are exceedingly slight; for instance, the 

 minimum stimulus in the form of green light is equal in terms of work to that which 

 is done in raising a ten-millionth part of a milligramme to the height of a millimetre, 

 and even some of this is doubtless wasted in the form of heat. The time during 

 which the stimulus acts may be excessively small, thus light from a rapidly rotating 

 mirror is visible even when 'it only falls upon the retina for one eight-millionth part 

 of a second. Some physiologists have drawn an analogy between retinal and 

 muscular excitations. There is no complete analogy, but the following points of 

 resemblance may be noted : 



1. The retina like the muscle possesses a store of potential energy, which the 

 stimulus serves to fire off. 



2. Fatigue on action, and recovery after rest are noticeable in both. 



* Purkinje's figures can be much more readily seen by simply looking steadily 

 down a microscope, and moving the whole instrument backwards and forwards, or 

 from side to side, while so doing. 



