NEUKO-MUSCULAK MECHANISM 



129 



to the right for the right eye, or to the left for the left eye, 

 when the point will disappear and again reappear (fig. 60, b). 

 (Practical Physiology.) The eye is blind for all objects in the 

 shaded region. By resolving the various triangles the distance 

 of the blind spot from the central spot of the eye may be 

 determined (3 to 4 mm.), and the diameter of the blind spot 

 (1'5 mm.) may also be ascertained. 



(2) The Field of Vision. The rest of the retina forward to 

 the edge is capable of stimulation, and the whole range of 

 objects which can be seen at one time con- 

 stitutes the field of vision, and it may be 

 indicated by the optical angle subtended by 

 that range of Objects. As the distance from 

 the eye increases the field of vision extends. 

 (3) The layer of the retina capable of 

 stimulation is the layer of rods and cones. 

 This is proved by the experiment of Pur- 

 kinje's images, which depends upon the fact 

 that if a ray of light is thrown through 

 the sclerotic coat of the eye the shadow of 

 the blood vessels stimulates a subjacent 

 layer (fig. 61, c), and these vessels appear 

 as a series of wriggling lines on the surface 

 looked at. If the light be moved, the lines 

 seem to move, and, by resolving the tri- 

 angles, it is possible to calculate the distance 

 behind the vessels of the part stimulated, 

 and this distance is found to correspond to 

 the thickness of the retina. The shadows 

 of the blood vessels are not seen in ordinary 

 vision, because they then fall upon parts of 

 the retina which are insensitive. (Practical Physiology.) 



The cones are the more specialised elements of the retina, 

 and they react more particularly to bright light, which soon 

 exhausts the rods. The rods, again, react to faint illumination. 

 This explains why it is that, when we go out into a dark night 

 from a brightly lighted room, we ab first can see nothing, but 

 after a time, when the rods have recovered, we begin to see 

 objects more distinctly. It also explains why it is that if we 

 direct our eye to a small faintly shining star we may fail to see 



9 



FIG. 61. To show that 

 the hindmost layer 

 of the retina is stim- 

 ulated. (Purkinjes 

 Images.) a, source 

 of light ; b, blood 

 vessel of retina ; c, 

 shadow of vessel on 

 rods and cones ; d, 

 image of shadow 

 mentally projected 

 on to the wall. 



