LXXII.] DIRECT VISION. 341 



9. Shadows of the Fovea Centralis and Retinal Blood-Ves<?e!s. 



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 out- 

 lines 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 the fovea centralis. Move the card vertically, when the 

 horizontal vessels are more 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. 



In a dark room light a candle, and stand in front of a mono- 

 chromatic 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, 

 downwards and laterally from the eye, 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 somewhat red- 

 brown branching lines, shadows of the retinal vessels, will be seen 

 on a red background, due to the shadows cast by the retinal 

 vessels on the percipient parts of the retina. Therefore the parts 

 of the retina stimulated by light must lie behind the retinal blood- 

 vessels. If the candle be moved in a vertical plane, the shadows 

 move upwards or downwards with the light. If the light be moved 

 horizontally, the shadows move in an opposite direction. 



Entoptical Vision. By this is meant the visual perception of 

 objects situated within our own eye. There are many such 

 phenomena. 



11. Muscse 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. 



(ft.) Rays of light proceeding from a point at or preferably within the 

 anterior focus of the eye, i.e., 13 mm. or less from the cornea, cast a shadow 

 of any object within the eyeball, because the rays fall parallel on the retina. 



Make a pin-hole in a card, place it close to the eyeball, and through the hole 

 look at an illuminated surface, e.g., a white lamp-shade, or white sky. The 

 margins of the aperture become luminous, i.e., they are the luminous body. 



