CHAP. m.J SIGHT. 919 



the opposite direction to the light when the latter is moved up 

 and down. In Fig. 153, which represents a horizontal section of 

 an eye, if a be moved to a, b (the illuminated spot on the retina, 

 the light reflected from which casts a shadow of v on to c) will 

 move to /3, the shadow on the retina c to 7, and the image d to 8. 

 If on the other hand a be supposed to move above the plane of 

 the paper, b will move below, in consequence c will move above, 

 and d will appear to move below, i.e. d will sink as a rises. 



FIG. 153. DIAGRAM ILLUSTRATING THE FORMATION OF PURKINJE'S FIGURES 

 WHEN THE ILLUMINATION is DIRECTED THROUGH THE CORNEA. 



It is desirable in these cases to keep moving the light to and 

 fro, especially in the first method, since the retina soon becomes 

 tired, and the image fades away. To give rise to a conscious 

 sensation of the slight difference between shadow and absence 

 of shadow the retina must be extremely sensitive ; if the shadow 

 remains motionless, the sensitiveness rapidly decreases in the 

 parts which are not in" shadow, until the visual sensations from 

 these parts are no stronger than those from the parts in shadow ; 

 when the light is moved the parts which were in shadow, not 

 having been so much stimulated, are sufficiently sensitive to 

 the light which now falls on them, while those parts which had 

 been previously fatigued recover their sensitiveness by resting 

 in the shadow. The experiment, like the experiment by which 

 the yellow spot ( 571) is made visible, is incidentally useful 

 as shewing how extremely sensitive and how soon fatigued are 

 the retinal structures. 



Some observers can recognize in the axis of vision a faint 

 shadow corresponding to the edge of the depression of the fovea 

 centralis. 



The retinal vessels may also be rendered visible by looking 

 through a small orifice such as a pin-hole in a card placed close 

 to the eye, in the position of the principal anterior focus, at a 

 bright surface such as a white cloud, and moving the orifice 

 very rapidly from side to side or up and down. If the move- 

 ment be from side to side, the vessels which run vertical will be 

 seen ; if up and down, the horizontal vessels. In this case, as in 



