826 



THE SENSE OF SIGHT 



image closer to the cornea, rendering it at the same time more globular. 

 These observations may be repeated under more favorable conditions 

 by making use of a darkened triangular box, known as the phacoscope 

 (Helmholtz). The eye to be observed is placed in the orifice at A 

 (Fig. 433) and is directed alternately to a needle situated in orifice B 

 and to a distant object placed in the prolongation of this visual line. 

 Orifice C is beset with two prisms which throw a beam of light into 

 the observed eye. The observer's eye studies these images through 

 orifice D. They appear as indicated in Fig. 434. 



Another most instructive phenomenon is the so-called wabbling 

 of the lens, l which consists in a declination of the lens on forced near 



vision of from 0.28 to 0.3 mm. This 

 phenomenon clearly proves that ac- 

 commodation diminishes the tension 

 under which the lens is held, allow- 

 ing its weight to force it out of the 

 central axis of the eyeball. The 

 direction of this declination depends 



D 



B 

 i 



FIG. 433. FIG. 434. 



FIG. 433. DIAGRAM ILLUSTRATING COURSE OF THE RAYS THROUGH THE PHACOSCOPE. 

 A, observed eye; B, opening allowing accommodation for near and far objects; 

 C, source of light; D, observer's eye. 1, images from cornea; 2, anterior surface of lens; 

 3, posterior surface of lens. 



FIG. 434. DIAGRAM OF REFLECTED IMAGES AS SEEN IN PHACOSCOPE. 

 A, during far vision; B, on near vision; 1, image from cornea; 2, image from anterior 

 surface of the lens; 3, image from posterior surface of the lens. 



of course upon the position of the head. Thus, when in the erect 

 position, near vision would allow the lens to drop downward com- 

 mensurate with the degree of accommodation. Subjectively we ob- 

 serve this phenomenon only under unusual conditions, for example, 

 when endeavoring to form a focus of those shadows which are ordi- 

 narily produced by opaque bodies floating through the aqueous or 

 vitreous humor. These particles then appear to execute jerky motions 

 in space. 



In addition, it might be mentioned that the mechanism of accom- 

 modation may be altered by drugs. The mydriatics atropin, homa- 

 tropin and cocain paralyze the ciliary muscle simultaneously with the 



1 Hess, Archiv fur Ophthalm., xliii, 1897, 477. 



