THE GLOBE OF THE EYE 803 



Refraction by a Biconcave Lens.— To understand the refraction 

 by biconcave lenses, imagine the lens to be composed of a number of 

 prisms, which in cross-section have their apices directed toward the 

 center or axis of the lens and their bases toward the periphery. If 

 we remember that the rays entering the incUnation of a prism, are 

 deflected toward its base, it must be evident that a biconcave lens 

 renders the rays divergent (Fig. 419). Like the concave mirrors, 

 these lenses give rise to virtual images. When the incident ray meets 

 the anterior surface of this lens, it is refracted toward the perpendicular, 

 CBj but away from it at H. This double refraction also takes place 

 with every other ray, for example, with DE and hence, there is no real 

 focus established. The prolongations of these rays intersect in F 

 which is the principal virtual focus. 



CHAPTER LXVIII 



THE GLOBE OF THE EYE AND ITS PROTECTIVE 

 APPENDAGES 



The General Structure of the Eyeball. — The eyeball is placed in 

 the fore part of the orbital cavity and is adjusted in such a way that it 

 may be activated by almost any ray projected toward it. Its range is 

 greatly increased by the fact that it may be moved in different direc- 

 tions by means of muscles attached to its external coat. In the mam- 

 mals, the visual mechanism consists of two eyeballs and their connec- 

 tions with the centers for vision in the occipital cortex of the cerebrum. 

 This implies that these animals are in possession not only of a most 

 highly developed receptor, but also of the means of forming the best 

 possible concepts. In this regard they are sharply differentiated from 

 the lower forms which, although equipped with receptors of sufficient 

 sensitiveness toward the ethereal impacts, are quite unable to asso- 

 ciate them properly, because they lack the central organ essential for 

 this function. Many of the lower forms are able to perceive Hght by 

 means of their pigment spots and other cutaneous sense-organs,^ 

 but react toward it merely in a reflex way, by displaying phenomena 

 similar to the heliotropism or phototaxis of the lowest organisms. 

 In a way, these forms are really in the same position as we would be 

 if our eyeHds were kept permanently closed, because although still 

 able to appreciate differences in the intensity of the illumination, we 

 would then react in accordance with these and no longer depend 

 upon distinct visual impressions and concepts. 



A much more advanced state of development is attained by the 

 eye of the higher invertebrates. That of the insects is composed of 



^ Hesse, Das Sehen der niederen Tiere, Jena, 1908. 



