270 THE HUMAN BODY 



window-shade. . Hold the candle to the side of one eye and close 

 to it, but so far back that no light enters the pupil from 

 it; that is, so far back that the flame just cannot be seen, but 

 so that a strong light is thrown on the white of the eye as far back 

 as possible. Then move the candle a little to and fro. The sur- 

 face looked at will appear luminous with reddish-yellow light, 

 and on it will be seen dark branching lines which are the shadows 

 of the retinal vessels. Now in order that these shadows may be 

 seen the parts on which the light acts must be behind the vessels, 

 that is, in the layers of the retina next the choroid since the blood- 

 vessels lie in its front strata. 



If the light be kept steady the vascular shadows soon disappear; 

 in order to continue to see them the candle must be kept moving. 

 The explanation of this fact may readily be made clear by fixing 

 the eyes for ten or fifteen seconds on the clot of an " i " somewhere 

 about the middle of this page : at first the distinction between the 

 slightly luminous black letters and the highly luminous white 

 page is very obvious; in other words, the different sensations 

 arising from the strongly and the feebly excited areas of the 

 retina. But if the glance be not allowed to wander, very soon 

 the letters become indistinct and at last disappear altogether; 

 the whole page looks uniformly grayish. The reason of this is 

 that the powerful stimulation of the retina by the light reflected 

 from the white part of the page soon fatigues the part of the 

 visual apparatus it acts upon; and as this fatigue progresses the 

 stimulus produces less and less effect. The parts of the retina, 

 on the other hand, which receive light only from the black letters 

 are but little stimulated and retain much of their original ex- 

 citability, so that, at last, the feebler excitation acting upon 

 these more irritable parts produces as much sensation as the 

 stronger stimulus acting upon the fatigued parts; and the letters 

 become indistinguishable. To see them continuously we must 

 keep shifting the eyes so that the parts of the visual apparatus are 

 alternately fatigued and rested, and the general irritability of 

 the whole is kept about the same. So, in Purkinje's" experiments 

 if the position of the shadows remain the same, the shaded part 

 of the retina soon becomes more irritable than the more excited 

 unshaded parts, and its relative increase of irritability makes up 

 for the less light falling on it, so that the shadows cease to be per- 



