350 



PRACTICAL PHYSIOLOGY. 



[LXXIV. 



10. Apparent Movements. 



(a.) Strobic Discs. Give the discs a somewhat circular but rapid movement 

 and observe that the rings appear to move, each one on its own axis. 



(6.) Radial Movement. 

 While another person rotates 

 a disc like fig. 271 on the 

 rotating wheel, look steadily 

 at the centre of the disc. 

 One has the impression as 

 if the disc were covered with 

 circles which, arising in the 

 centre and gradually becom- 

 ing larger, disappear at the 

 periphery. After long fixa- 

 tion look at printed matter 

 or at a person's face ; the 

 letters appear to move 

 towards the centre, while 

 the person's face appears 

 to become smaller and re- 

 cedo. If the disc be rotated 

 in the opposite direction, 

 the opposite results are ob- 

 tained. 



(c.) Fix an object, turn 

 the head rapidly, and note 

 that the object appears to 

 move in an opposite direction. "When the eye does not move, we judge 

 that a body is in motion when the image of that body falls successively on 

 different points of the retina, and at the same time are conscious that the 

 ocular muscles have not contracted (Beaunis). 



Fio. 271. 



LESSON LXXIV. 



KUHNE'S ARTIFICIAL EYE MIXING COLOUR 

 SENSATIONS COLOUR-BLIN DNESS. 



1. KUhne's Artificial Eye (fig. 272). 



(a.) Fill the instrument with water, and place it in a darkened room with 

 the cornea directed to a hole in a shutter, through which sunlight is directed 

 by means of a heliostat. If this is not available, use an oxy-hydrogen lamp 

 or electric light to throw parallel rays of light on the cornea. If these cannot 

 be had, use a fan-tailed gas-burner, but in this case the illumination and 

 images will be feeble. To enable one to observe the course of the rays of light, 

 pour some eosin or fluorescin into the water in the instrument. 



(b.) Formation of an image on the retina. Observe the course of the rays 

 of light, which come to a focus behind the lens the principal posterior focus. 

 Move the ground glass representing the retina, and get a clear inverted image 

 of the source of light. N.B. In this instrument accommodation is effected 

 not by altering the curvature of the lens, as in the normal eye, but by moving 

 the retina. 



