en. LVL] 



ACCOMMODATION 



781 



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 flame 

 (fig. 582). 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 surfacef'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 the eye is now adjusted for 

 a far point, the second image enlarges again, 

 becomes less distinct, and recedes from the 

 first. In both cases the first and third 

 images remain unaltered in size, distinct- 

 ness, and position. This proves that during 

 accommodation for near objects the curva- 

 ture of the cornea, and 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 is more striking when two bright images (repre- 

 sented by arrows in fig. 583) are used ; the two images from the front 



FIG. 582. 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 sur- 

 face of lens. 



Fir; 583. 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. 



surface of the lens during accommodation not only approach those 

 from the cornea, but also approach one another, and become some- 

 what smaller. (Sanson's Images). Helmholtz's Phakoscope (fig. 584) 

 is a triangular box with arrangements for demonstrating this 

 experiment. 



Mechanism of Accommodation. The lens having no inherent 

 power of contraction, its changes of outline must be produced by 



