CHAP, in.] SIGHT. 113 



down. In Fig. 149, 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 /?, 

 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. 149. 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 ( 768) 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 movement 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 the 

 similar instance of shadows cast by objects in the vitreous humour 



8 



