300 On the Mechamfm of the Eye, 



<b adapted, that the magnitude of the confufed image may remain perfetlly conflant. 

 Indeed, to calculate from the dimenfions which we have hitherto ufed, it would be ex- 

 f e£ted that the image fliould be diminiflied about one-fortieth, by the utmoft increafe of 

 the convexity of the lens. But the whole depends on the fituation of the refracting fiwr- 

 faces, and the refpe£tive incieafe of their curvature, which, on account of the variable 

 denfity of the lens, can fcarcely be eftimated with fufEcient accuracy. Had the pupil been 

 placed before the cornea, the magnitude of the image muft, on any fuppofition, have been 

 very variable: at prefent, this inconvenience is avoided by the fituation of rhe pupil; f© 

 that we have here an additional inftance of the perfection of this admirable organ. 



From the experiments related, it appears to be highly improbable that any material 

 change in the length of the axis a£lually takes place ; and it is almoft impoiBble to conceive 

 by what power fuch a change could be efFefted. The ftraight mufcles, with the adipofe 

 fubflance lying under them, would certainly, when a£ting independently of the focket, 

 tend to flatten the eye : for, fince their contraction would neceflarily leflen the circum- 

 ference or fuperficies of the mafs that they contain, and round off all its prominences, 

 their attachment about the nerve and the anterior part of the eye muft therefore be brought 

 nearer together. (Plate XV. Fig. 21, 22.) Dr. Olbers compares the mufcles and the eye 

 to a cone, of which the fides are protruded, and would by contraction be brought into a 

 ftraight line. But this would require a force to preferve the cornea as a fixed point, at a 

 given diftance from the origin of the mufcles ; a force which certainly does not exift. In 

 the natural fituation of the vifual axis, the orbit being conical, the eye might be fomewhat 

 lengthened, although irregularly, by being forced further into it ; but, when turned to- 

 wards either fide, the fame a£lion would rather fliorten its axis ; nor is there any thing 

 about the human eye that could fupply its place. In quadrupeds, the oblique mufcles are 

 wider than in man ; and in many fituations might affift in the eflfeCt. Indeed a portion of 

 the orbicular mufcle of the globe is attached fo near to the nerve, that it might alfo co- 

 operate in the aftion : and I have no reafon to doubt the accuracy of Dr. Olbers, who 

 ftatesi that he efFe6ted a confiderable elongation, by tying threads to the mufcles, in the 

 eyes of hogs and of calves ; yet he does not fay in what pofition the axis was fixed ; and 

 the flaccidity of the eye after death might render fuch a change very eafy as would be im- 

 poffible in a living eye. Dr. Olbers alfo mentions an obfervation of Profeflbr Wrifberg, on 

 the eye of a man whom he believed to be deftitute of the power of accommodation in 

 his life-time, and whom he found, after death, to have wanted one or more of the mufcles: 

 but this want of accommodation was not at all accurately afcertained. I meafured, in 

 the human eye, the diftance of the attachment of the inferior oblique mufcle from 

 the infertion of the nerve : ic was one-fifth of an inch ; and from the centre of vifion not 

 a tenth of an inch; fo that, although the oblique mufcles do in fome pofitions nearly form 

 a part of a great circle round the eye, their adion would be more fitted to flatten than to 

 *Mtlongate it. We have therefore reafoa to agree with Winflow, in attributing to them the 



the 



