ACCOMMODATION. 499 



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 increased, for example by 

 compression of the eyeball by the muscles around it; how- 

 ever, experiment shows that changes of accommodation can 

 be brought about in the fresh excised eyes of animals, 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 more 

 diverging rays sooner to a focus; since 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 sur- 

 face of the lens becomes considerably 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 (p. 493). If, 

 therefore, a person be taken into a dark 

 room and a candle held on one side of his 

 eye, while he looks at a distant object an 

 observer can see three images of its flame h 11 media 

 in his pupil, due to that part of the light 

 reflected from the surfaces between the media. One (a, 

 Pig. 132) is erect and bright, reflected from the convex 

 mirror formed by the cornea; the next, 1 9 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. If now the 

 observed eye looks at a near object in the same line as the 

 distant point previously looked at, it is seen that the image 

 due to corneal reflection remains unchanged; that due to 

 light from the front of the lens becomes smaller and brighter, 

 indicating (see Physics) a greater convexity of the reflecting 

 surface; the image from the back of the lens also becomes very 

 slightly smaller, indicating a feebly increased curvature. 



