PHOTORECEPTION 199 



light in such a cylinder is diagrammed in Fig. 94. As a ray xm enters 

 the cylinder it is refracted by the surface of each lamella a 'b\ but 

 since the lamellae decrease in refractive index progressively from A^to 

 ab, the ray is bent less and less until it re-enters regions of higher re- 

 fractive index, where its path is now directed back towards xy. It finally 

 emerges at^*. If such a cylinder is as long as its focal length it will pro- 



Fig. 94. Course followed by light through a cylinder abed corresponding to 

 the dioptric apparatus of the ommatidium. A, path of a ray of light. 

 B, course of a spherical wave, a -b', layer of equal refringence ; x, point 

 of origin of wave; mm, mini, etc.", successive positions of wave front 

 indicating changes due to refraction accompanying passage through 

 cylinder; y, focal point. (Redrawn from Exner, 1891.) 



duce an inverted image at its base. If a cyhnder is twice as long as its 

 focal length it will produce an inverted image at its midpoint (Fig. 95). 

 The rays will then diverge from the focal point and continue on straight 

 paths. Rays that enter the cylinder obliquely (e.g., from the right) will 

 emerge from the same side (i.e., the right). 



If, in the apposition eye, the ommatidium is the optical unit, if it 

 perceives Hght primarily from points on or near its axis, if there is 

 minimal overlap of ommatidial visual fields and the image which has 

 o 



