346 THE SPECIAL SENSES. 



shadows are thrown upon a portion of the retina not usually affec- 

 ted 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 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 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. 149. 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. 



