USE OF THE MICROSCOPE IN ZOOLOGY. 6 1 



little hexagonal figures, forming so many eyes, or ocelli, 

 as they are termed. In the common fly the two eyes 

 contain about 4,000 of these hexagonal facets, or ocelli. 

 The eyes of insects differ according to the species, 

 both in position, number, form, and colour. The eyes 

 of the common white butterfly are composed of about 

 17,000 ocelli; in the dragon-fly there are upwards 

 of 20,000. 



By making a very careful vertical section you will 

 discover that each ocellus is in shape like a pyramid ; 

 the upper part, or corneule, forming the base, the apex, 

 or lower part, which is drawn to an extremely fine 

 point, coming in contact with some delicate extremi- 

 ties of nerve-fibres which branch out from the optic 

 nerve. It has been shown that each corneule is a 

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

 plano-convex lenses possessing a different refractive 

 power, by which arrangement, probably, the aberra- 

 tions are diminished, as they are by the combination of 

 "humours" in the human eye. "That each 'corneule' 

 acts as a distinct lens may be shown by detaching 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 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 pyramids, which consist of a transparent sub- 

 stance, representing, it is supposed, the " vitreous hu- 

 mour," are separated from each other by a layer of 

 dark pigment, which at one point closes in, but leaves 

 very minute papillary apertures for the entrance of rays 

 from the corneule, which, passing down the pyramids, 



* " The Microscope," p. 662. 



