174 



THE EYE IN EVOLUTION 



b 



Fig. 159. — The 

 Lens Cone of 

 THE Compound 

 Eye. 



The laminated 

 optical structure of 

 superimposed la- 

 mellae (after Exner). 



emerge as a diverging beam (m' p' n'). Oblique rays (g) will 

 emerge at an angle as q' . The pigment mantle around the 

 cones, however, will absorb oblique rays and virtually 

 permit the light to emerge only at y, where the image falls 

 as a single luminous point on the subjacent retinule ; the 

 apposition of all such points will form the complete erect 

 image perceived by the eye. 



In the superpositional eye, on the other hand, the 

 optical system will correspond to a lens-cylinder of a length 

 equal to twice its focal distance (Fig. 161). The inverted 

 image of a distant object will be formed in the middle of the 

 cylinder {xy) ; the rays traversing the remaining half of the 

 cylinder will pursue a symmetrical course and emerge at an 

 angle (^) equal to that at which they entered (a) but 

 opposite in direction. Not only will normal rays thus fall 

 on the distant rhabdome but also oblique rays refracted 

 from the cones towards the same side from which they have 

 come, so that a number of separate images can be super- 

 iinposed on one visual element. The resultant image thus 

 gains in luminosity at the sacrifice of resolution. 



The ability to analyse the plane of polarized light is 

 a common function of the compomid eyes of Arthropods and of both the 

 simple and compound eyes of Insects ; it is a function which is freely 

 used to aid orientation out-of-doors.^ The structure which serves as 

 an analyser, however, has given rise to controversy. The suggestion that 



Fig. 160. — The Optical System of 

 THE Apposition Eye. 



The i; u^ cylinder is equal in length 

 to its foe- distance (after Exner). 



Fig. 161. — The Optical System of 

 THE Superposition Eye. 



The lens cylinder is equal in length 

 to twice its focal distance (after Exner). 



1 p. 66. See Kalmus. Nature (Lend.), 184, 228 (1959). 



