Visual Organs of Insects a?id Crustacea. 



371 



situated in the direction of the axes of the cones, it follows, 

 if double vision is to be avoided, that the bases of the cones 

 in the two eyes ought never to be so placed as to have a direc- 

 tion convergent towards each other ; for otherwise the same 

 object may be seen by both eyes, and, in consequence of pro- 

 bable inequality of distance, it may appear in different parts 

 of the two fields of vision. If, '\iijig> 68., a b is parallel to 



c df the curves 

 there deline- 

 ated will serve 

 to indicate all 

 the possible 

 forms met with 

 in nature, as 

 it regards the 

 respective po- 

 sition of the 

 compoundeyes 

 If the eyes 

 form two hemi- 

 spheres, as in the Hemerobius p^rla, the diameters a h and 

 c d o{ these are always so placed as to be either parallel, or 

 divergent from each other in front. In the former case, a o 

 and 6 c must be regarded as the axes of those cones placed 

 most in front, and nearest to the inner margins of the two 

 eyes : they indicate, in consequence, the inner boundaries of 

 the two fields of vision in front ; and, of course, the object 

 that is visible to one eye must be invisible to the other. Few 

 insects have perfectly hemispherical eyes; but, when their 

 surface is spherical, they always constitute either the whole, 

 or segments, of the hemispheres a h and c d; and, in all such 

 cases, the prolonged sector radii show the boundaries of the 

 two fields of vision to be completely distinct. Thus, when the 

 eye, by its position and size, coincides w4th m a o, the lines m o 

 and a o form the boundaries of the field of vision ; for the eye 

 nao the boundaries are formed by the lines n o and a o ; and 

 for the eye 6 p q^ by the lines 6 q and 6 p. In no known in- 

 stances are the hemispheres, or smaller segments of spheres, 

 extended in front and towards each other beyond the parallel 

 lines a b and c d, for example, to e andy; for, in this latter 

 case, objects placed in a direction within the visual angles 

 a e and c ojl would almost inevitably be visible to both 

 eyes, and would be seen as double. 



The lines a7/b and c /r d enclose segments of ellipses with 

 the greater curvature a little in front : this is observed in the 

 eyes of many insects, as in the genera Mantis, Lema, Doria- 



B B 2 



