156 EXPERIMENTAL PHYSIOLOGY. 



refracting surfaces. By this means spherical aberration is cut down 

 in near vision, in which great accuracy is usually required and in 

 which the aberration would otherwise be marked, because the 

 incoming rays are very divergent. The image is made somewhat 

 less bright by this device, since there are fewer rays to contribute 

 to it than there would be with a wider pupil, but, the object being 

 near, the number of rays which the eye receives from it is in any 

 case large and the loss is not of much importance. All three groups 

 of muscles which contract in near vision, the ciliary, the circular 

 muscle fibres of the iris, and the internal recti, are supplied by the 

 oculomotor nerve. The nerve supply of the radial fibres of the 

 iris, the contraction of which dilates the pupil, comes from the 

 cervical sympathetic.* 



Some indication of the nature of the change in the eye media 

 during accommodation may be got from the following experiments. 

 Experiment 60. In a dark room arrange the lens and watch glass 

 so that they are centred, with the lens behind the watch glass, 

 which is placed with its convex side forward. Hold a candle 

 beside your head and a little behind your eyes and look at" the 

 images of the flame formed by the three refracting surfaces. 

 These let most of the light through, but reflect some small 

 portion of it. The reflected rays form the images which you 

 see. There are three of them, one inverted and two erect. 

 Experiment 61. Purkinje Images. Look at your partner's 

 eye in the way in which you looked at the lens and watch glass 

 in the last experiment and ask the subject to gaze past you into 

 the distance, -along a line half way between your eye and the 

 candle. As before, three images may be seen but in this case 

 one of the erect images is larger and dimmer than the other. 

 Now ask the subject to focus on your finger held in his line of 

 vision, at about the distance of your head away. The small, 

 bright, erect image and the inverted one are unchanged in 

 position, but the larger erect one moves nearer the first and 



*A stopping-down of the pupil, similar to that which occurs for near vision, is 

 brought about reflexly when the eye moves from a darker to a lighter place. If 

 the change in the intensity of the light is great the contraction is more marked at 

 first, becoming gradually less for some minutes afterwards as the eye becomes 

 'accustomed to the light". This is known as the "LIGHT REFLEX" of the pupil. 



