PROPERTIES OF THE RETIXA. 



345 



the retinal images into consciousness. Some of these entopic ap- 

 pearances are described briefly, but for a detailed description the 

 reader is referred to the classical work of Helmholtz.* 



The Blood-corpuscles. The entoptic images that are most easily 

 recognized perhaps are those of the moving corpuscles in the capil- 

 laries of the retina. If one looks off into the blue sky he will have 

 no difficulty in recognizing a number of minute clear and dark specks 

 that move in front of the eye in definite paths. The character of 

 the movement leaves no doubt that these sensations are due to the 

 shadows of the blood-corpuscles. In fact, the shadows often show 

 a rhythmic acceleration in velocity synchronous with the heart- 

 beats, a pulse movement. By projecting the moving images upon 

 a screen at a known distance from the eye the velocity of the capil- 

 lary circulation has been estimated in man. 



The Retinal Blood-vessels. The blood-vessels of the retina lie 

 in front of the rods and cones and must necessarily throw their 

 shadows upon these sensitive end-organs. The shadows may be 

 made more distinct and a visual picture of the vessels obtained by 

 a number of methods. For instance, if a card with a pin hole 

 through it is moved slowly in front of the eye the images of the 

 blood-vessels stand out in the field of vision with more or less 

 distinctness. The card should be given a circular movement. If it 

 is kept in one position the images quickly disappear, since the 

 retina apparently fatigues very quickly for such faint impressions. 

 A more impressive picture may be obtained by the method of 

 Purkinje. In a dark room one holds a candle toward the side of the 

 head in such a position as 

 to give the sensation of a 

 glare in the correspond- 

 ing eye. If the eye is di- 

 rected toward the opposite 

 side of the room and the 

 candle is kept in continual 

 circular movement the 

 blood-vessels appear in 

 the field of vision magni- 

 fied in proportion to the 

 distance of projection; the 

 picture makes the impres- 

 sion of a thicket of inter- 

 lacing branches. In this experiment the light from the candle 

 strikes the nasal side of the retina at an oblique angle and is re- 

 flected toward the other side of the globe. The blood-vessels are 

 in this way illuminated from an unusual direction and their 



* Helmholtz, " Handbuch der physiologischen Optik," second edition, 

 I, 184. 



a 



Fig. 149. Helmholtz's method of showing en- 

 toptic phenomena due to imperfections in the lens 

 and vitreous (Helmholtz) : c, a screen with pinhole ; 

 a, lens with short focus. 



