THE EYE AS AN OPTICAL INSTRUMENT 261 



we look at the one we see the other only indistinctly ; and if, after- 

 looking at the more distant object, we look at the nearer we expe- 

 rience a distinct sense of effort. It is clear, then, that something 

 in the eye is different in the two cases. The resting eye, suited for 

 seeing distinctly distant objects, might conceivably be accommo- 

 dated for near vision in several ways. The refracting indices of its 

 media might be increased; that of course does not happen; the 

 physical properties of the media are the same in both cases: or the 

 distance of the retina from the refracting surfaces might be in- 

 creased, for example, by compression of the eyeball by the muscles 

 around it; however, experiment shows that changes of accommoda- 



cs 



cb 



FIG. 88. Diagram to illustrate the mechanism of accommodation; on the 

 right half of the figure for a near, on the left for a distant, object; rf, ciliary muscle; 

 ch, ciliary process of choroid; si, suspensory ligament; i, iris. 



tion can, by stimulating the third cranial nerve, be brought about 

 in the fresh excised eyes of animals from which the muscles lying 

 outside the eyeball have been removed, in which no such compres- 

 sion is possible; we are thus reduced to the third explanation, that 

 the refracting surfaces, or some of them, become more curved, and 

 so bring diverging rays sooner to a focus. Observation shows 

 that this is what actually happens: the corneal surface remains 

 unchanged when a near object is looked at after a distant one, 

 but the lens becomes considerably more convex. 



Accommodation is brought about by the ciliary muscle (Fig. 88) . 

 In the resting eye it is relaxed and the suspensory ligament of the 

 lens is taut, and, pulling on its edge, drags it out laterally a little 

 and flattens its surfaces, especially the anterior, since the ligament 



