172 ON THE POWER OF VISION IN INSECTS. 



On the other hand, Clapavede asserts that at a 

 distance of twenty feet a hive bee would be unable to 

 see any object which was less than eight or nine inches 

 in diameter, and even at a distance of a foot he says 

 that each facet would correspond to an inch and a third. 



To determine how far a faceted eye could see, he 

 takes the breadth of a facet, the radius of the eye- 

 sphere, and the smallest angle of vision, and the dis- 

 tance in centimetres at which the facet would cover 

 a centimetre, and finds for the bee, for instance, 6 '7 

 centimetres. 



He then proceeds to inquire at what distance from 

 the faceted eye the image is as clear as in the human 

 eye, and he thinks this would be about a millimetre, 

 from which it would rapidly diminish, being only -f^ at 

 a centimetre, and at a metre no distant vision being 

 possible; so that at a very little distance such eyes 

 would be as good as useless. 



"In the human eye, for example, the distance 

 between the centres of two adjacent cones is only 

 2Q%ij mm., but in Musca the distance between adjacent 

 ommatidia is jl^ mm. In fact, the picture, as received 

 by the nerve-end cells of the Vertebrate eye, is much 

 more complete in itself than it can possibly be in any 

 Arthropod eye, and consequently the latter possesses 

 a much more elaborate and complete translating appa- 

 ratus in its retina than the former possesses." * 



Claparede arrives at this conclusion by taking 

 the average curvature of the whole eye, as being true 

 for each part. This, however, is not the case, and 

 in the central region of the eye the adjacent facets 



* S. J. Hickson, " The Eye and Optic Tract of Insects," Quarterly 

 Juurnal of Microscopical Science, vol. xxv., new series, 1885, p. 242. 



