554 0" *i" Mechamfm of the Eye. 



(^'g- 57') ^^^t wRen viewed in a microfcope, the tranfverfe divifions appear to be cracks 

 and the whole mal's is evidently of a uniform tgxture, without the Icaft fibrous appearance; 

 and» if a particle of any kind of inufcle is compared with it, the contrail becomes very 

 ftriking. Befides, it is fixed down, throughout its extent, to the pofterior lamina of the 

 choroid, and has ho attachment capable of dire£ling its efrecf; to fay nothing of the diffi- 

 culty of conceiving what that efFe£t could be. Its ufe rauft remain, in common with that 

 ©f many other parts of the animal frame, entirely concealed from our curiofity. 

 .pj;'ifhc;bony fcalea of the eyes.of birds, which were long ag9 deftribed in the Philofophica} 

 Ti!anfa<^ions by Mr, Ranby *, and by Mr. Warren t, afterwards in two excellent Memoirs 

 ojf M. Petit, on the eye of the turkey and of the owl J, and lately by Mr. Pierce Smiih ^, 

 ^j)4:Mr» Home if, can, on any fuppofition, have but little concern, in tl^ accommodation of 

 tlie eye to different diftances : they rather fee.Ti to be neceffary for the protection of that 

 organ, large and prominent as it is, and unfupported by any ftrength in the orbit, againfl; 

 the various accidents to which the mode of life and rapid motion of thofe animals m.ufl ex- 

 pofe it ; and they are much, lefs liable to fracture thau an entire bony ring of the fame thick- 

 nefs would have been. The marfupium nigrum appears to be intended to afiift in giving 

 ftrength totheeye, to prevent any change in the place of the lens Iw external force: it is 

 £} fituated as to intercept but little light, and that little is principally what would have 

 {^Ilen on the infertion of the optic nerve ; and it feems to be too firmly tied to the lens, 

 ^ven to admit any confiderable elongation of the axis of tlie eye, although it certainly would 

 not impede a protrufion of the cornea. 



With refpecl to the eyes of infe£is, an obfervation of Poupart deferves to be repeated 

 here. He remarks, that the eye of the libellula is hollow -, that it communicates wich an 

 air-veffel placed longitudinally in the trunk of the body ; and that it is capable of being in- 

 flated from this body : he fuppofes that the infedt is provided with this apparatus, in order 

 for the accommodation of its eye to th.e perception of objedls at different dillances **. I 

 have not yet had an opportunity of examining the eye of the libellula; but there is no dif- 

 ficulty in fuppofing that the means of producing the change of the refracTiive powers of the 

 eye, may be, in different clafTes of animals, as divetfified as their habits, and the general 

 conformation of their organs. 



I beg leave to correal here an obfervation in my- former paper, relative to the faint lateral 

 radiations, which L fuppofcd to proceed from the margin of the iris ft. I find, ou further 

 examination, that they are occafioncd by reflections from the eye-laflies. 



XII. I fhall now finally recapitulate the principal obje£ts and refults of the invelf igation 

 which, I have taken the liberty of detailing fo fully to the Royal Society. Firft, the deter- 



* Pkil. Tranr. Vol. XXXIII. p. 223. Abr.Vol.VII. p. 435. 

 t Phil. Tranf. Vol. XXXIV. p. 113. Abr. Vol. VII. p. 437. 

 J Mem. de rA<:ad. 1735, p. 163- 1736, p. 166. Ed. Amft. 



§ Phil. Tranf. for 1793, p. 263. J Phil. Tranf, for 1796, p. It. 



* • Phil. Tranf. Vol. XXII. p. 673. Abr. II, p. 762. 

 tt Phil* Tranf. fer 1793, p. 178. 



2 mination 



