SENSE OF SIGHT. 567 



cornea. Besides this image there is a second image (6), which 

 is also erect, but which is less distinct and larger ; this image 

 is formed at the anterior surface of the lens. A third image (c) 

 is also seen, which is inverted and also indistinct ; this image is 

 formed by the posterior surface of the lens, which, being concave 

 forward, acts like a concave mirror and inverts the image. These 

 are called Purkinje-Sanson images. If the person then looks as if 

 at a near object, the second image becomes brighter and smaller, and 

 at the same time approaches the first, while the first image under- 

 goes no change, and the third a change so slight as not to be per- 

 ceptible (Fig. 343). This proves that in accommodating the eye 

 for near objects the principal change which takes place is an in- 

 crease in the convexity of the anterior surface of the crystalline 

 lens. There is also a slight increase in the convexity of the 

 posterior surface, while the cornea remains unchanged. In Fig. 

 344 the course taken by the rays of light is delineated. The eye 

 whose accommodation is under investigation is directed to A, while 



FIG. 345. Phakoscope of Helmhokz: at B B' are two prisms, by which the light 

 of a candle is concentrated on the eye of the person experimented with ; A is the 

 aperture for the eye of the observer. The observer notices three double images 

 reflected from the eye under examination when the eye is fixed upon a distant 

 object ; the position of the images having been noticed, the eye is then made to focus 

 a near object, such as a reed pushed up ; the images from the anterior surfaces of the 

 lens will be observed to move toward each other, in consequence of the lens becom- 

 ing more convex. 



the candle-flame and the observer's eye are on opposite sides. The 

 images of the candle-flame will appear along the line IF, on the 

 dark background of the pupil. The image produced by reflection 

 from the cornea is seen at the termination of the dotted line a ; 

 that from the posterior surface of the lens, at the termination of 

 c ; and that from the anterior surface of the lens when the eye is 

 in a state of accommodative rest i. e. 3 looking at distant objects 



