230 THE MICROSCOPE AND ITS REVELATIONS. 



eyes ' are examined under even a low magnifying power, by the ' facetted ' 

 appearance of the surface (Fig. 423, A), which is marked-out by very 

 regular divisions either into hexagons or into squares: each facet is the 

 'corneule ' of a separate ocellite, and has a convexity of its own; hence by 

 counting the facets, we can ascertain the number of ocelli tes in each ' com- 

 pound eye. ' In the two eyes of the common Fly, there are as many as 

 4,000; in those of the Cabbage Butter-fly there are about 17,000; in the 

 Dragon-fly, 24,000; and in the Mardella Beetle, 25,000. Behind each 

 'corneule' is a layer of dark pigment, which takes the place and serves 

 the purpose of the 'iris' in the eyes of vertebrate animals; and this is 

 perforated by a central aperture or ' pupil,' through which the rays of 

 light that have traversed the corneule gain access to the interior of the 

 eye. The further structure of these bodies is best examined by vertical 

 sections (Fig. 424); and these show that the shape of each ocellite (b) is 

 conical, or rather pyramidal, the corneule forming its base (a), whilst its 

 apex abuts upon the extremity of a fibre (c) proceed- 

 FIG. 425. ing from the termination of the optic nerve (d). The 



details of the structure of each ocellite are shown in 

 Fig. 425; in which it is shown that each corneule is 

 a double-convex lens, made up by the junction of two 

 plano-convex lenses, a a and a' a', which have been 

 found by Dr. Hicks to possess different refractive 

 powers; by this arrangement (it seems probable) the 

 aberrations are diminished, as they are by the combina- 

 tion of ' humors ' in the Human eye. That each ' cor- 

 neule ' acts as a distinct lens, may be shown by de- 

 taching the entire assemblage by maceration, and then 

 drying it (flattened out) upon a slip of glass; for when 

 this is placed under the Microscope, if the point of a 

 knife, scissors, or any similar object, be interposed 

 Minute structure of between the mirror and the stage, the image of this 

 P oint wil1 beeen, bv a proper adjustment of the focus- 



. 

 comeuie; a' a', its pos- of the microscope, in every one of the lenses. The 



^Ip^ures's^l focus of each corneule ' has been ascertained by 

 ed by intervening pig- experiment to be equivalent to the length of the pyra- 



mentd d; bb, pyramids -i i i -i , 11 



separated by pigment mid behind it; so that the image which it produces 



ee, < bJ&1ig a SSt2! wil1 fal j u P n the . extremity of the filament of 

 of nerve-fibres. the optic nerve which passes to the latter. The 



pyramids (1), b) consist of a transparent substance,. 

 which may be considered as representing the 'vitreous 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 substance. 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 reach the fibres of the optic nerve. Hence, it 

 is evident, that, as no two ocellites on the same side (Fig. 424) have ex- 

 actly the same axis, no two can' receive their rays from the same point of 

 an object; and thus, as each compound 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 



