THE EYE AS AN OPTICAL INSTRUMENT. 523 



a lace curtain, at a window, we can as we choose look at 

 the threads of the lace or at the houses across the street; but 

 when 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 experience a distinct sense of elfort. It is clear, 

 then, that something in the eye is different in the two cases. 

 The resting eye, suited for distinctly seeing distant objects, 

 might conceivably be accommodated for near vision in several 

 ways. The refracting indices of its media might be in- 

 creased; that of course does not happen; the physical prop- 

 erties of the media are the same in both cases: or the dis- 

 tance 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 accommodation 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 re- 

 moved, in which no such compression 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; for a lens of smaller curvature is more 

 converging than one of greater curvature composed of the 

 same material. 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 anterior surface of the lens becomes con- 

 siderably more convex and the posterior 

 slightly so. As already pointed out, when 

 light meets the separating surface of two 

 media some is reflected and some refracted. 

 If, therefore, a person be taken into a 

 dark room and a candle be held on one side 

 of his eye while he looks at a distant object, a J e s - 

 an observer can see three images of the flame {^Sed fronHhe re- 

 in his pupil, due to that portion of the light tracting media of the 

 reflected from the surfaces between the 

 media. One image (a, Fig. 150) is erect and bright, reflected 

 from the convex mirror formed by the cornea; the next, b, is 

 dimmer and also erect; it comes from the front of the lens. 

 The third, c, is dim and inverted, being reflected from the 

 concave mirror (see Physics) formed by the back of the lens. 

 When the curvature of a curved mirror is altered the size of 



