THE EYE AS AN OPTICAL INSTRUMENT. 205 



power of adjustment or 'accommodation' is necessary. 

 This is accomplished by the movements 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 con- 

 tracts, the tension of the lens is diminished, and its sur- 

 faces — 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 increased, 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 'accommodated' 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 prac- 

 tical 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 reflexions of light through 

 the pupil from the two surfaces of the crystalline lens, 



