COMPOUND EYES OF INSECTS. 



663 



Fig. 340. 



similar object, be interposed between the mirror and the stage, 

 the image of this point will be seen, by a proper adjustment of the 

 focus of the microscope, in every one of the lenses. The focus of 

 each ' Corneule ' has been ascertained by experiment to be equi- 

 valent to the length of the pyramid behind it ; so that the image 

 which it produces will fall upon the 

 extremity of the filament of the Optic 

 Nerve which passes to the latter. 

 The pyramids (6, b) consist of a trans- 

 parent substance, which may be con- 

 sidered as representing the ' Yitreous 

 Humor ;' and they are separated from 

 each other by a layer of dark pigment, 

 d', d', which closes-in at d, d, between 

 their bases and the corneules, leaving 

 a set of Pupillary apertures, c, c, for the 

 entrance of the rays which pass to them 

 from the ' corneules. ' After traversing 

 these pyramids, the rays reach the 

 bulbous extremities e, e of the fibres of 

 the Optic Nerve, which are surrounded, 

 like the pyramid, by pigmentary sub- 

 stance. Thus the rays which have 

 passed through the several ' Corneules ' 

 are prevented from mixing with each 

 other ; and no rays, save those which 



pass in the axes of the Pyramids, can separated by intervening 

 reach the fibres of the Optic Nerve. Pigment dd; b b, Pyramids 

 Hence it is evident, that, as no two 

 Ocelli on the same side have exactly 

 the same axis, no two can receive their 

 rays from the same point of an object ; 



and thus, as each Composite Eye is immovably fixed upon the 

 Head, the combined action of the entire aggregate will probably 

 only afford but a single image, resembling that which we obtain 

 by means of our Single eyes. — Although the foregoing may be 

 considered as the typical structure of the Eyes of Insects, yet 

 there are various departures from it (most of them slight) in the 

 different members of the Class. Thus in some cases the posterior 

 surface of each ' Corneule ' is concave ; and a space is left be- 

 tween it and the Iris-like diaphragm, which seems to be occupied 

 by a watery fluid or ' Aqueous Humor ; ' in other instances, again, 

 this space is occupied by a double-convex body, which seems to 

 represent the 'Crystalline-Lens;' and this body is sometimes 

 found behind the Iris, the number of Ocelli being reduced, and 

 each one being larger, so that the cluster presents more resemblance 

 to that of Spiders, &c. — Besides their Composite Eyes, Insects 

 usually possess a small number of rudimentary Single Eyes, 



Minute structure of the 

 Eye of the Bee: — a a, an- 

 terior lenses of Corneule ; 

 a' a', its posterior lenses ; 

 c c, Pupillary apertures, 



separated by Pigment d'd', 

 and abutting on e e, bulb- 

 ous extremities of Nerve- 

 fibres. 



