276 EXPERIMENTAL PHYSIOLOGY. 



Press any other part of the eyeball, the same spectrum is 

 seen, and always on the opposite side. Impressions made 

 on the terminations of the optic nerve are referred outside 

 the eye i.e., beyond into space. The phosphene is seen in 

 the upper half if the lower is pressed, and vice versd. 



9. Shadows of the Fovea Centralis and Retinal Blood-vessels. 



(a.) Move, with a circular motion, a blackened card, with 

 a pin-hole in its centre, in front of one eye looking through 

 the pin-hole at a white cloud. Soon a punctated field 

 appears with the outlines of the capillaries of the retina. 

 The oval shape of the yellow spot is also seen, and it will be 

 noticed that the blood-vessels do not enter ihefovea centralis. 

 Move the card vertically, when the horizontal vessels are 

 most distinct. On moving it horizontally, the vertical ones 

 are most distinct. Some observers recommend that a slip of 

 blue glass be held behind the hole in the opaque card, but 

 this is unnecessary. 



10. Purkinje's Figures. 



(a.) Darken a room, light a candle, and stand in front of 

 a monochromatic wall. If this is not available, hang up a 

 large white sheet, and while looking steadily with one eye 

 towards the wall or sheet, accommodating the eye for a 

 distant object, hold the candle close to the side of that eye, 

 well out of the field of vision, and move the candle up and 

 down. It is better to direct the eye outwards, keeping it 

 accommodated for a distant object. Ere long, dark some- 

 what red-brown branching lines, shadows of the retinal 

 vessels, will be seen on a dark background. Therefore the 

 parts of the retina stimulated by light must lie behind the 

 retinal blood-vessels. 



11. Muscae Volitantes. 



(a.) Light a candle in a dark room; at a distance from it 

 place a black screen with a pin-hole in it. Focus by means 

 of a convex lens the image of the flame upon the hole in 

 the screen. Look through the hole with one eye, and on 

 the illuminated part of the lens will be seen images of dots 

 and threads due to objects within the eyeball. 



