PROPERTIES OF THE RETINA. 



367 



retina at an oblique angle and is reflected 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 por- 

 tion of the retina not usually affected and for that reason perhaps 

 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 simi- 

 lar relatively opaque ob- 

 jects in the lens may 

 throw shadows on the 

 retinal bottom. These 

 shadows take different 

 forms, but usually are de- 

 scribed as small spheres 

 or beads, single or in 

 groups, that move with 

 the eyes and are desig- 

 nated, therefore, as the 

 muscae 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. 156. 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. 



Fig. 156. 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. 



