6"02 



ox THE MECHANISM OF THE EYE. 



ment of the case, in his valuable work on com- 

 parative anatom3' ; and bis descriptions, as 

 well as those of Svvamnierdam, agree in ge- 

 neral with what I have observed. We are 

 prejudiced in favour of their being eyes, by 

 their situation and general apjjearance. The 

 copious supply of nerves seems to prove, at 

 least, that they must be organs of sense. In 

 the hermit crab, Swamiiierdam says, that 

 their nerves even decussate, but this is not 

 the case in the crawfish. The external 

 coat is always transparent; its divisions are 

 usually more or less leniiculur. Many insects 

 have no other organs at all resembling eyes; 

 and wImju these eyes have been covered, the 

 insects appear to have been either wholly 

 or partially blinded*. Hut, on the other 

 hand, nnany insects are without tliese eyes, 

 and of tiiose who have them, many have 

 others also, more unquestionably fitted for 

 vision. The neighbouring parts of the hard 

 skin or shell are often equallv tr;inspa- 

 rent with these, when the crust lining them 

 is removed. In the apis longicornis, the an- 

 tennae, as Mr. Kirby first iiifornved me, have 

 somewhat of the same reticulated appearance 

 but not enough for the foundation of any 

 argument respecting its use. This reticu- 

 lated coat is always completely lined by an 

 obscure and opaque mucus, which appears 

 perfectly unfit for the transmission of light ; 

 nor is there any thing like a transparent hu- 

 mour in the whole structure: and the con- 

 vexity of the lenticular portions is bv no 

 means sufticiently great, to bring the rays of 

 light to a very near focus; indeed, in lobsters, 

 the cxteruid surface is perfectly equable, and 

 tlic internal surface is only divided into 

 squares by a cancellated texture adhering to 

 iU There is nothing in any way analogous 



• Hooke Microgr. ijg. 



to a retina, and there can be no formation 

 of such an image, as is depicted in the eyes 

 of all other animals, not excepting even the 

 vermes: nor does there appear to be room to 

 allow with Bidloo that there is a perforation, 

 admitting light, under the centre of each 

 hexagon. If they are eyes, their manner of 

 perceiving light must rather resemble the 

 sense of hearing than that of seeing, and they 

 must convey but an iinperfect idea of the 

 form of objects. And it maybe remarked 

 that beetles, which have no other eyes, fly 

 much by night, and are proverbially dull- 

 sighted. The stemniata, which are usually 3, 

 6, 8, or 12 in number, have much more in- 

 disputably the appearance of eyes. In the 

 wasp, they consist externally of a thick 

 double convex lens, firmly fixed in the shell, 

 perfectly transparent, an<l externally very 

 hard, but internally softer; behind this ap- 

 pears to be a vitreous hutnour, and probably 

 behind that, there is a retina. Here we 

 must consider the crystalline lens as united 

 to the cornea, without any uvea or aqueous 

 humour. In the reticulated eyes, there is 

 nothing resembling a. crystalline lens. The 

 stemmata have never any motion, but they 

 are capable of comprehending, conjointly, a 

 very extensive field of view ; and it is possi- 

 ble that the j)osterior part of the lens may 

 have a power of changing its convexity for 

 the perception of objects at different dis- 

 tances. 



XI. I shall now finally recapitulate the 

 principal objects and results of the investiga- 

 tion, which I have taken the liberty of detail- 

 ing so fully to the Royal Society. First, the 

 det'ermitvuiooof tiie refractive power of a va- 

 riable medium, (M.E. 465.)and itsapplication 

 to the constitution of the crystalline lens. Se- 

 condly, the construction of ao instrument for 



