200 THE PHYSIOLOGY OF INSECT SENSES 



significance for the insect is a mosaic of point of light, then visual 

 acuity should depend upon the number and spacing of luminous points 

 received from the object in the visual field. There should be exact 

 correspondence between the minimum angle subtended by adjacent 

 ommatidia and the minimum angle subtended by two points that are 

 perceived as separate (the reciprocal of the visual angle is termed the 

 visual acuity). 



■-/' 



Fig. 95. Course of light rays through a lens cylinder abed whose length is 

 (A) equal to its focal length (B) twice the focal distance, m, p, n, q, 

 rays from objects in the visual field; q', n', p', m', course of rays after 

 passage through cylinder; xy, optic axis; zy, inverted image of object 

 at base of cylinder. Note that in B an inverted image is produced at 

 mid-point of cylinder. Rays continue through to produce an erect 

 image at the rhabdoms. (Redrawn from Exner, 1891.) 



Acuity does indeed depend upon the angle between adjacent direc- 

 tional elements, but reducing this angle to increase acuity has limita- 

 tions. There is no improvement in acuity when the angle is so close that 

 visual overlapping occurs, and there is now evidence that such over- 

 lapping is much greater than would be predicted from Muller's theory 

 (de Vries, 1956; Burtt and Catton, 1954). Furthermore, as Barlow 

 (1952) has pointed out, visual acuity is related to the resolving power of 

 each ommatidial lens. 



The resolving power of a lens is limited by its optical properties and 

 the wave nature of light. The diameter of the ommatidium is critical, 

 and diffraction must be taken into consideration. Because of dif- 



