ORGANS OF SPECIAL SENSE THE EYE 345 



is like ordinary glass lenses, though more convex on both sides. 

 If taken out of the eye, the lens still has enough rigidity of its 

 own to keep its shape. It is in the same cavity with the vitre- 

 ous humor in a vertical plane just back of the iris. Note in 

 Figure 167 that the lens is more convex on the back than on 

 the front side, and that at its edges it is held in place by slender 

 ligaments which run outward into the choroid layer. These are 

 collectively called the suspensory ligament, which must be 

 thought of as a thin sheet of ligamentous tissue going entirely 

 about the lens border, rather than as a cord attached to any 

 one or several points. It swings the lens into position, by 

 tension on its edges in every direction. 



THE FORMATION OF IMAGES 



When one's eyes are open, there is formed upon the retina 

 a picture or image of the object in front of the eye. The 

 secret of this picture formation lies entirely in the lens and 

 the cornea, more especially in the former. 



Everyone knows that light, reflected from trees, buildings, 

 people or other objects, on passing through the lens of a photo- 

 grapher's camera produces an impression on the sensitive 

 plate within. If we compare this plate to the retina of the 

 eye, and the lens to the lens of the eye, the similarity between 

 the eye and the camera is very striking. In Figure 167 is 

 shown the arrangement of the lens and the retina. Straight 

 lines show the direction which rays of light take in the eye. 

 In this case it is supposed that the object seen is a point and 

 that the rays of light entering the eye are parallel. It will 

 be seen that these rays after passing through the lens bend 

 from their parallel direction and come together. It is plain 

 that they must meet, otherwise they would strike many 

 different parts of the retina at the same time, and a large 

 number of points instead of one would be seen. Each would 

 be seen indistinctly, too, since the light would be so sub- 

 divided as not to be intense at any one place on the retina. 



