346 THE SENSORIAL FUNCTIONS. 



It thus appears that each of the constituent eyes, which 

 compose this vast agji;regate, consists of a simple tube, fur- 

 nished with all the elements requisite for distinct vision, 

 and capable of receivinsj; impressions from objects situated 

 in the direction of the axis of the tube. The rays tra- 

 versing adjacent corneules are prevented from mixing them- 

 selves with those which arc proper to each tube by the in- 

 terposition of the black pigment, which completely surrounds 

 the transparent cylinders, and intercepts all lateral or scat- 

 tered light. Thus has nature supplied the want of mobility 

 in Ihc eyes of insects, by the vast multiplication of their 

 number, and by providing, as it were, a separate eye for 

 each separate point which was to he viewed; and thus has 

 she realized the hypothetical arrangement, which suggested 

 itself in the outset of our inquiries, while examining all the 

 possible modes of eficcting this object. 



This mode of vision is probably assisted by the converging 

 powers of each corneule, although in parts which are so mi- 

 nute it is hardly possible to form an accurate estimate of 

 these powers by direct experiment. In corroboration of this 

 view I am fortunately enabled to cite a valuable observation 

 of the late Dr. Wollaston, relative to the eye of the Aslacus 

 Jhiviatilis, or cray-fish, where the length of each compo- 

 nent tube is short, compared with that of the Libellula. On 

 measuring accurately the focal distance of one of the cor- 

 neules. Dr. Wollaston ascertained that it corresponds with 



filaments (f) of the optic nerve, in tlieir progress inwards, as passing through 

 a second membrane (k, Fig. 428,) which he denominates the common cho- 

 roid, and afterwards uniting to form an expanded layer, or more general re- 

 tina (h,) whence proceed a small number of short but thick nervous co- 

 lumns (>',) still converging towards the large central ganglion (g,) in which 

 they terminate. The use he ascribes to this second clioroid is to intercept 

 the light, which, in so diminutive an organ, might otherwise penetrate to 

 the general retina and produce confusion, or injurious irritation. The co- 

 lour of the pigment is not always black, but often has a bluish tint: in the 

 common fly, it is of a bright scarlet hue, resembling blood. In nocturnal 

 insects the transvci-sc layer of pigment between the corneule and the vitre- 

 ous humour is absent. 



