!66 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



FIG. 106. — Diagram section through 

 compound eye : C, cephalic ganglion. 



passes down the corresponding tube and impresses its 

 own point on the retina, and thus forms the image. Rays 



passing into other tubes 

 strike the pigmented 

 sides and are quenched, 

 and thus mixing is pre- 

 vented, but with great 

 loss of light. 



Comparison with Sim- 

 ple Eyes. — Comparing 

 now with normal eyes 

 of invertebrates, we see 

 great differences in sev- 

 eral respects, i. In or- 

 dinary eyes distinctness 

 is reached by bringing 

 all the rays from each 

 radiant to a single focal 

 point on the retina. In this method, on the contrary, 

 the same result is secured, but with great loss of light, 

 by allowing only the central rays from each radiant to 

 reach and impress the retina. 2. In all other eyes, ver- 

 tebrate or invertebrate, the image is inverted; in this, 

 on the contrary, it is erect (Fig. 106). Nevertheless, in 

 this case also, by the law of direction, the object is seen 

 erect. The reason is that in all other eyes the recipient 

 surface is concave, and therefore reinverts the image in 

 the act of external reference, while in this it is convex, 

 and does not reinvert the image. 3. In vertebrate eyes 

 a wide field is got by free motion of the eye in its 

 socket; but in compound eyes it is got by the sphericity 

 of the large surface. In Crustacea, where the sphericity 

 is less great, the eye is placed on the end of a movable 

 stalk. It is probable that the sight of the compound 

 eye is very imperfect except at short distance. 



