THE COMPOUND EYE 



173 



Exner's early work on the dioptrics of the ommatidial system still remains 

 classical. He showed that the essential refractive device is the crystalline cone, 

 which, of course, vmlike the lens of Cephalopods and Vertebrates, has a fixed 

 focus incapable of adjustment. The crystalline cone itself is composed of 

 concentric lamellae the refractive index of which increases progressively from 

 the perii^hery to the central axis (Fig. 159) ; it therefore acts as a " lens-cylinder " 

 wherein an obliquely incident ray is progressively refracted until it is gradually 



Forficiila 



Fig. 158. — The Ommatidial Angles of the Eye of the Honey-Bee. 



The ommatidia are drawn in groups of 3, and the drawing shows the 

 way in which an ommatidial angle varies in different parts of the eye ; the 

 values of the angles are given in degrees (Pirenne, after Baumgartner). 



brought back to the axis. It is probable that the crystalline cone thus brings 

 the image formed bj^ an ommatidimn to a small point although different wave- 

 lengths will be brought to a focus at different places (Goulliart, 1953). To some 

 extent therefore, the optics of the comjaoand eye with its many elements is 

 comparable to the analysis made by television. 



The appositional eye wherein the retinule abuts against the crystalline 

 cone may be compared oj)tically to such a system wherein rays of light pass 

 through a lens-cylinder of a length equal to its focal distance (Fig. 160). In this 

 event a beam of parallel light (mpn) entering perpendicularly to one edge of the 

 cylinder (ab) will be focused as an inverted image at y on the other edge and will 



Locust a 



