VISION 165 



arrangement of the pigment and the sensory elements deter- 

 mines that each element is stimulated from a particular direc- 

 tion. The image in this case is formed by projection. If 

 there is a cuticular lens it is too close to the retina to form an 

 image; it only serves to concentrate the light. 



384. When the sensory elements are arranged on a convex 

 surface as in the insect eye, an image is also formed by pro- 

 jection, but in this case the image is erect while in the concave 

 eye it is inverted. However, the cuticular lens and the cone 

 of the ommatidium are so placed that an image is formed in 

 the plane of the retinal cells. There is, therefore, a combina- 

 tion of image by projection and image by refraction. This 

 type of eye is comparatively efficient. Form is distinguished 

 with considerable detail, and colors are recognized, but there 

 is still a deficiency which makes the insect eye decidedly in- 

 ferior to the vertebrate eye. There is no provision for focusing. 

 It is possible that the great depth of the sensory element (the 

 rhabdom) is in some measure a compensation; the multiplicity 

 of eyes is another. In some cases a single eye is so constructed 

 that one part is adapted for far vision, the other for objects 

 near at hand. 



385. In some Vertebrates (fishes) the eye is focused by mov- 

 ing the lens toward or away from the retina. A more refined 

 method is adopted by the higher Vertebrates. The lens is 

 elastic and is continually flattened somewhat by the pressure 

 of the capsule on the anterior and posterior surfaces of the lens. 

 By the contraction of the ciliary muscles this pressure is some- 

 what removed and the lens, by elasticity, assumes a more con- 

 vex form. 



386. In many eyes the quantity of light admitted to the 

 sensory elements is controlled by movements in the pigment 

 cells. When the light is too intense the pigment advances 

 and cuts off some rays. In weak light the pigment recedes, 

 thus admitting a broader beam of light. This adjustment is 



