364 THE SPECIAL SENSES. 



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 

 1 „^^^ groups, that move with 



Fig. 156.— Helmholtz's method of showing en- the eyeS and are clcsig- 



toptic phenomena due to imperfections in the lena „ ,„ i .i „j?„,.„ „„ +u„ 



and vitreous (i^e/m/iote): c, a screen with pinhole; Rated, thoretore, aS tllO 



a. lens with short focus. musciB volitantcs (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 smafl, bright image of the flame at that spot. The shadows 

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



Colored Rings Around Luminous Sources. — Bright hghts seen in 

 the dark, street lamps, for example, may appear to be surrounded 

 by a colored ring the size of which varies with the distance of the 

 light. This phenomenon is described sometimes as a symptom of 

 glaucoma, but it is apparently present in the normal eye when the 

 conditions are suitable.* The ring is blue on the inside and red 

 on the outside, and in the center there may be a luminous glow, 

 the ciliary corona, surrounding the source of illumination. The 

 ciliary corona seems to be due to the lens fibers, while the colored 

 ring is possibly a diffraction phenomenon due to the cells of the 

 anterior epithelial layer of the cornea or to the endothelium of 

 Descemet's membrane (Sheard). 



* Sheard, "American Journal of Physiological Optics," July, 1920, 267. 



