ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



495 



function of vision still required investigation. The close parallel 

 with the Vertebrate eye which was attempted to be drawn, is quite 

 fallacious. Clearness of sight was said by Joh. Miiller to coincide 

 with long sight, aud to be best exhibited in those eyes which have 

 the greatest circumference — the greatest number of very small facets, 

 large crystalline cones and dark pigment-mass ; the nearer the object 

 to the eye, he said, the clearer the view obtained of it. 



Dr. J. Notthaft believes the size of the facets and the length of 

 the radius of the curve of the eye to be the only factors in its 

 structure which have an important bearing on this point. With 

 regard to the effect produced by the presence of a number of receptive 

 and refractive units — the units of the compound eye — he believes 

 that the edges of the units and fields of sight are in contact ; when 

 the curve of the eye is perfectly spherical the fields are approxi- 

 mately polyhedric, like the facets, but when the curve is eccentric, 

 distortion appears as magnification increases. The smallest angle of 

 sight is constituted by the angular distances between the directions 

 in which two neighbouring retinal elements look, or even between 

 two such ocular elements regarded as wholes. A few examples may 

 be given of the actual condition of things in some specific eyes : — 



* Determined by calculation from the radius of the eye-sphere as compared with the breadth 



of a facet. 



In spite of its large minimum angle of vision, the insect eye 

 affords, under certain circumstances, as great an amount of distinct- 

 ness of vision as the human eye, or even greater ; for there is no 

 minimum limit to the distance of vision, and objects near the eye 

 are seen more clearly than anywhere else ; the distinctness of vision 

 diminishes as the square of the distance from the eye : these relations 

 for the following insects are : — 



