824 



THE EYE AND VISION 



[CH. LVIIL 



experience; that the mind localises objects with reference to its 

 own body, and that from the first it knows nothing of the inversion 

 of the retinal image, as its powers of localisation only appear with 

 developing general experience. 



ACCOMMODATION 



The power of accommodation is primarily due to an ability to 

 vary the shape of the lens ; its front surface becomes more or less 

 convex, according as the distance of the object looked at is near or 

 far. The nearer the object, the more convex, up to a certain limit, 



the front surface of the lens becomes, and 

 vice versd ; the back surface takes no share 

 in the production of the effect required. 

 The posterior surface, which during rest is 

 more convex than the anterior, is thus ren- 

 dered the less convex of the two during 

 accommodation. The following simple ex- 

 periment illustrates this point : If a lighted 

 candle be held a little to one side of a 

 person's eye an observer looking at the eye 

 from the other side sees three images of the 

 CftunTthe anterior surface" of flame (fig. 524). The first and brightest is 



lens; 3, from the posterior sur- /*\ fi , iiji 



face of lens. (1) a small erect image formed by the an- 



terior convex surface of the cornea; the 



second (2) is also erect, but larger and less distinct than the pre- 

 ceding, and is formed at the anterior convex surface of the lens; 

 the third (3) is smaller, inverted, and indistinct; it is formed at 

 the posterior surface of the lens, which is concave forwards, and 

 therefore, like all concave mirrors, gives an inverted image. If 

 now the eye under observation is made to look at a near object, the 

 second image becomes smaller, clearer, and approaches the first. If 



FIG . 525. Diagram of Sanson's images. A, When the eyes are not, and B, when they are focussed for 

 near objects. The fig. to the right in A and B is the inverted image from the posterior surface of 

 the lens. 



the eye is now adjusted for a far point, the second image enlarges 

 again, becomes less distinct, and recedes from the first. In both 



