PROPERTIES OF THE RETINA. 



337 



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 

 corresponding eye. If the eye is directed 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 magnified in proportion 

 to the distance of projection; the picture makes the impression of a 

 thicket of interlacing 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 shadows are thrown 

 upon a portion of the ret- 

 ina not usually affected 

 and for that reason per- 

 haps more sensitive to 

 the impression. 



Imperfections in the Vitreous Humor and the Lens. Small frag- 

 ments of the cells from which the vitreous humor was constructed 

 in the embryo and similar relatively opaque objects in the lens may 

 throw shadows on the retinal bottom. These shadows take 

 different forms, but usually are described as small spheres or beads, 

 single or in groups, that move with the eyes and are designated, 

 therefore, as the muscse volitantes (flitting flies or floating flies). To 

 bring out these shadows it is convenient to make the source of illu- 

 mination small and to bring it at or nearer than the anterior focal 

 distance of the eye (15 to 16 mms.). The method employed for this 

 purpose by Helmholtz is illustrated in Fig. 147. In this figure b 

 is a candle flame, and a a lens of short focus which makes an image 

 of the flame at the small opening shown in the dark screen, c. The 

 eye is placed just behind this opening and is illuminated by the rays 

 from the small, bright image of the flame at that spot. The shadows 

 are seen projected upon the illuminated surface of the glass lens. 

 22 



Fig. 147. Helmholtz's method of showing en- 

 toptic phenomena due to imperfections in the lena 

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

 a, lens with short focus. 



