THE SENSES. 



597 



of four inches, is on] y about 0.143 of an inch. 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 maintain the same form, 

 would not be more than about one line. 



It is now almost universally believed that Helmholtz is right in his 

 statement that the immediate cause of the adaptation of the eye for ob- 

 jects at different distances is a varying shape of the lens, its front surface 

 becoming more or less convex, according to the distance of the object 

 looked at. The nearer the object, the more convex does the front sur- 

 face of the lens become, and vice versd; the back surface taking little or 

 no share in the production of the effect required. The following simple 

 experiment illustrates this point. If a small flame be held a little to one 



FIG. 404. 



FIG. 405. 



Fro. 404. Diagram showing three reflections of a candle. 1, From the anterior surface of cor- 

 nea; 2, from the anterior surface of lens; 3, from the posterior surface of lens. For further ex- 

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

 Helmholtz, termed a Phakoscqpe. 



FIG. 405. -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 Fig. 404, 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 by C ; the images from 

 the anterior surfaces of the lens will "be observed to move towards each other, in consequence of 

 the lens becoming more convex. 



side of a person's eye, an observer looking at the eye from the other side 

 sees three distinct images of the flame (Fig. 404). The first and bright- 

 est 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 and reversed, it is formed at the posterior surface 

 of the lens, which is concave forwards, and therefore, like all concave 



