714 HANDBOOK OF PHYSIOLOGY. 



ference between the focal distances of the images of an object at a distance, 

 and of one at the distance of four inches, is only about 0.143 of an inch 

 (3.5 mm.). On this calculation the change in the distance of the retina 

 from the lens required for vision at all distances, supposing the cornea 

 and lens to remain the same, would not be more than about one line. 



The adaptation of the eye for objects at different distances is pri- 

 marily due to a varying shape of the lens, its front surface becoming 

 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, and vice versa; the back surface tak- 

 ing little or no share in the production of the effect required. And this 

 surface, which during rest is more convex than the anterior, becomes 

 the less convex of the two during accommodation. The following simple 

 experiment 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 distinct images of the flame (fig. 423). The first and 

 brightest is (1) a small erect image formed by the anterior convex surface 

 of the cornea; the second (2) is also erect, but larger and less distinct than 

 the preceding, 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 forward, and therefore, 

 like all concave mirrors, gives an inverted image. If now the eye under 

 observation be made to look at a near object, the second image becomes 

 smaller, clearer, and approaches the first. If the eye be now adjusted 

 for a far point, the second image enlarges again, becomes less distinct, 

 and recedes from the first. In both cases alike the first and third images 



Fig. 423. Diagram showing three reflections of a candle. 1, From the anterior surface of 

 cornea ; 2, from the anterior surface of lens ; 3, from the posterior surface of lens. For further 

 explanation, see text. The experiment is best performed by employing an instrument invented 

 by Helmholtz, termed a Phakoscope. 



remain unaltered in size, distinctness, and relative position. This 

 proves that during accommodation for near objects the curvature of the 

 cornea, and of the posterior of the lens, remains unaltered, while the 

 anterior surface of the lens becomes more convex and approaches the 

 cornea. 



