534 THE HUMAN BODY. 



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 but little stimulated and retain much of their 

 original excitability, so that, at last, the feebler excitation act- 

 ing upon these more irritable parts produces as much sensa- 

 tion as the stronger stimulus acting upon the fatigued parts; 

 and the letters become indistinguishable. To see them con- 

 tinuously 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 experiment, 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 perceived. It is 

 for this reason that we do not see the retinal vessels under ordi- 

 nary circumstances. When light, as usual, enters the eye 

 from front through the pupil the shadows always fall on the 

 same parts of the retina, and these parts are thus kept suffi- 

 ciently more excitable than the rest to make up for the less light 

 reaching them through the vessels. To see the latter we 

 must throw the light into the eye in an unusual direction, 

 not through the pupil but laterally through the sclerotic. If 

 v, Fig. 158, be the section of a retinal 

 vessel, ordinarily its shadow will fall 

 at some point on a line prolonged 

 through it from the centre of the pupil. 

 If a candle flame be held opposite b it 

 illuminates that part of the sclerotic 

 and from there light radiates and illu- 

 mines the interior of the eye. The 

 resulting sensation we refer to light 

 entering the eye in the usual manner 

 through the pupil, and accordingly see 

 the surface we look at as if it were illuminated. The shadow 

 of v, is now cast on an unusual spot c, and we see it as if at the 



