780 



THE EYE AND VISION 



[CII. LVI. 



line lens ; the ray A C, which is parallel to the optic axis 0', is 

 refracted through the principal posterior focus P, and cuts the first 

 ray at the point A' on the retina. All the other rays from A meet 

 at the same point. Similarly the other end of the arrow B is focussed 

 at B', and rays from all other points have corresponding focusses. 

 It will thus be seen that an inverted image of external objects is 



Fio. 581. Diagram of the course of the rays of light, to show how an image is formed upon the retina. 

 The surface C C should be supposed to represent the ideal curvature. 



formed on the retina. The retina is a curved screen, but the images 

 fall only on a small area of the retina under normal circumstances ; 

 hence, for practical purposes, this small area may be regarded as flat. 

 The question then arises, Why is it that objects do not appear to 

 us to be upside down ? This is easily understood when we remember 

 that the sensation of sight occurs not in the eye, but in the brain. 

 By education the brain learns that the tops of objects excite certain 

 portions of the retina, and the lower parts of objects other portions 

 of the retina. That these portions of the retina are reversed in 

 position to the parts of the object does not matter at all, any more 

 than it matters when one's photograph arrives home from the 

 photographer's that it was wrong way up in the photographer's 

 camera one puts it right way up in the photograph album. 



ACCOMMODATION 



The power of accommodation is primarily due to an ability to 

 vary the shape of the lens ; its front surface becomes 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 becomes, and vice versd ; the back 

 surface takes no share in the production of the effect required. The 

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

 is thus rendered the less convex of the two during accommodation. 

 The following simple experiment illustrates this point : If a lighted 



