182 RECENT PROGRESS OF THE THEORY OF VISION. 



is screwed in. The eye has the same task of bringing at one 

 time near, at another distant, objects to a focus at the back of 

 its dark chamber. So that some power of adjustment or ' ac- 

 commodation ' is necessary. This is accomplished by the move- 

 ments of the crystalline lens (Fig. 28, L), which is placed a 

 short distance behind the cornea. It is covered by a curtain of 

 varying colour, the iris (J), which is perforated in the centre by 

 a round hole, the pupil, the edges of which are in contact with 

 the front of the lens. Through this opening we see through 

 the transparent and, of course, invisible lens the black chamber 

 within. The crystalline lens is circular, bi-convex, and elastic. 

 It is attached at its edge to the inside of the eye by means of a 

 circular band of folded membrane which surrounds it like a 

 plaited ruff, and is called the ciliary body or Zonule of Zinn 

 (Fig. 28, * *). The tension of this ring (and so of the lens 

 itself) is regulated by a series of muscular fibres known as the 

 ciliary muscle (Cc). When this muscle contracts, the tension of 

 the lens is diminished, and its surfaces but chiefly the front 

 one become by its physical property of elasticity more convex 

 than when the eye is at rest ; its refractive power is thus in- 

 creased, and the images of near objects are brought to a focus 

 on the back of the dark chamber of the eye. 



Accordingly the healthy eye when at rest sees distant objects 

 distinctly : by the contraction of the ciliary muscle it is ' ac- 

 commodated ' for those which are near. The mechanism by 

 which this is accomplished, as above shortly explained, was 

 one of the greatest riddles of the physiology of the eye since the 

 time of Kepler ; and the knowledge of its mode of action is of 

 the greatest practical importance from the frequency of defects 

 in the power of accommodation. No problem in optics has given 

 rise to so many contradictory theories as this. The key to its 

 solution was found when the French surgeon Sanson first observed 

 very faint reflections of light through the pupil from the two 

 surfaces of the crystalline lens, and thus acquired the character 

 of an unusually careful observer. For this phenomenon was 

 anything but obvious ; it can only be seen by strong side illumi- 

 nation, in darkness otherwise complete, only when the observer 



