644 



THE SENSES 



figure 457. The first and brightest is, i, 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 



FIG. 459. 



FIG. 458. Diagram of Sanson's Images. A, When the eyes are focussed for far objects, and 

 B, when they are focussed for near objects. The figure to the right in A and B is the inverted 

 image from the posterior surface of the lens. 



FIG. 459. Phakoscope of Helmholtz. At B, B', are two prisms, by which the light of a candle 

 is concentrated on the eye of the person experimented with at C. A is the aperture for the eye 

 of the observer. The observer notices three double images, as in figures 457 and 458, reflected from 

 the eye under examination when the eye is fixed upon a distant object; the position of the images 

 having been noticed, the observed eye is then focussed on a near object, such as a reed pushed up 

 by C; the images from the anterior surface of the lens will be observed to move toward each other, 

 in consequence of the lens becoming more convex. 



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 be- 

 comes 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 remain 

 unaltered in size, distinctness, and relative position. This proves that during 

 accommodation for near objects the curvature of the cornea, and that of the 

 posterior surface of the lens, remain unaltered, while the anterior surface of 

 the lens becomes more convex and approaches the cornea. 



The experiment, figure 458, is more striking when the two prisms of the 

 phakoscope which form two images of the candle are used. The pair of 

 images of the candle from the front surface of the lens not only approach 

 those from the cornea during accommodation, but also approach one another, 

 and become somewhat smaller, Sanson's images. 



