510 THE HUMAN BODY. 



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 outer layers of the retina since the vessels- 

 lie (p. 490) in its inner strata. 



If the light is kept steady the vascular shadows soon dis- 

 appear; 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 dot 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 do not be 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 very little stimu- 

 lated and retain their original excitability 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 indistin- 

 guishable. To see them continuously we must keep shift- 

 ing the eyes so that the parts of the visual apparatus 

 are alternately fatigued and rested, and the general irrita- 

 bility of the whole is kept about the same. So, in 

 Purkinje's experiment, if the position of the shadows 

 remain the same, the shaded part of the retina soon be- 

 comes 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 

 perceived. It is for this same reason that we do not see 

 the vessels under ordinary circumstances. When light, as 

 usual, enters the eye from front through the pupil the 



