VISION. 



237 



front, with nerve-terminations behind it, and bright pig- 

 ment round about : of the light entering the lens, no doubt 

 the rays corresponding in colour with the pigment are 

 reflected from point to point, and a sensation must thus be 

 produced varying in intensity with the amount of that parti- 

 cular colour of light; but it can scarcely be supposed that any 

 nearer approach to vision is made. 



Fig. 120. OCELLI OF STARFISH (solaster papposa). a, Pignient- 

 cone; 6, lens; c, c, nerve-corpuscles. H. S. "Wilson. 



Eyes which receive an image are divisible into two great 

 groups, those in which the image is erect, and those in which 

 ifc is inverted. To the first of these 

 groups belong the eyes of insects and of 

 crustaceans, such as lobsters; while to 

 the second group belong the eyes of ver- 

 tebrata and cuttlefishes. If the eye of a 

 lobster or a dragon-fly be carefully ex- 

 amined, it will be seen to consist of 

 numbers of minute facets, barely visible, 

 crowded together; each of these is a 

 transparent structure placed at the ex- 

 tremity of a long tube lined with black 

 pigment, and with a nerve-termination Fig. 121. Diagram 

 at its deep extremity. Obviously no of INSECT'S EYE in 

 ray of light can reach the bottom of section, 

 such a tube unless it fall vertically into it. Thus the 

 point in the landscape vertically opposite each tube affects 

 the nerve at its extremity, and a separate sensation is pro- 

 duced by as many points as there are facets in the eyes, and 

 this will happen irrespective of the distances of objects. 



In eyes which invert the image, the inversion is produced by 



