268 The Evolution of Optics. 



semi-fluid mass, the vitreous humor. Now we have all the 

 apparatus necessary for giving us distinct pictures of dis- 

 tant objects. 



Let us now look at the important mechanism by which 

 the eye is enabled to adjust its focus to both distant and 



FIG. 1. Horizontal section of the right eye (Landois). a, cornea ; 6, conjunctiva; 

 c, sclerotic; d, anterior chamber containing the aqueous humor; e, iris; //', 

 pupil; gr, posterior chamber; I, Petit's canal; j, ciliary muscle ; k, corne-oscle- 

 ral limit ; i, canal of Schlemm ; m, choroid; n, retina; o, vitreous humor; 

 No, optic nerve; q, nerve-sheaths; p, nerve- fibers; Ic, lamina cribrosa. The 

 line OA indicates the optic axis; Sr, the axis of vision; r, the position of the 

 f ovea centralis. 



near objects. For our purpose we may consider the diop- 

 tric apparatus of the eye as constituting one single lens, 

 although in reality the cornea, lens, and humors of the eye 

 each contribute their share to the result. We assume rays 

 of light coming from a distance that is, from more than 

 twenty feet to be parallel, and in a normal eye to be suffi- 

 ciently refracted to form a clear image of the desired ob- 

 ject upon the retina. 



Now, it is manifestly impossible that without some adjust- 

 ment this same apparatus should give us clear images of 

 nearer objects, because the rays of light from a nearer ob- 

 ject are more divergent and require a stronger refracting 



