OBJECTIVE AND SUBJECTIVE LIGHT 3 



this plan that the 'compound' eyes of many arthropods are constructed. 

 Vertebrate eyes are all built upon one fundamental plan. With the 

 exception of those which have degenerated because their owners live 

 underground, or in the perpetual night of caves or the depths of the 

 ocean, they are provided with a retina and with a lens whose optical 

 properties are such that it forms an image upon the retina. The lenses of 

 the median eyes which some reptiles possess on the top of the head are 

 probably often of the concentrator type; but those of the lateral, or 

 ordinary, eyes are nearly always eikonogenic — that is, image-forming. 



c n^ 



Fig. 1 — Various photoreceptors. 



a, intracellular type in a one-celled animal, Pouchetia cornuta. b, scattered photosensory 

 cells in the skin of an earthworm, c, pit-like visual organ of a limpet, Patella, d, pinhole- 

 camera type of eye in the chambered nautilus, e, ocellus of a scorpion, Euscorpius, with 

 concentrating lens, f, eye of a snail, Murex. g, image-forming eye of a squid, Loligo. 

 h, eye of vertebrate. 



c- cuticle; e- epithelium; /- lens; n- nerve fibers; p- pupil; r- retina; s- secreted material. 



Before we pass to a consideration of the detailed structure and work- 

 ings of a standard vertebrate eye, it needs to be further emphasized that 

 vision, seeing, is a phenomenon of the mind plus the eye and not of the 

 eye alone. It would probably not stagger any reader of this book to be 

 asked to believe that a worm may react to a light-stimulus without having 

 a sensation or consciousness of light. Vertebrate vision as we ourselves 

 experience it, however, is more than just photoreception. Vertebrate 

 visual mechanisms, from fish to mammal, are so nicely constructed that 

 so far as the eyes themselves are concerned, they may in many cases send 



