EYE. 



19* 



lulls of the numerous experiments of Petit lead 

 to the conclusion, that the anterior curvature is 

 that of a portion of a sphere from .six to seven 

 lines and a half in diameter, the posterior that of 

 a sphere of from five to six lines and a quarter. 

 From the same source it appears that the dia- 

 meter is from four lines to four lines and a half, 

 the axis or thickness about two lines, arid the 

 weight three or four grains. I am, however, 

 niflined to agree with the observation of Porter- 

 field, that, " as it is scarce possible to measure 

 the crystalline and the other parts of the eye 

 with that exactness that may be depended on, 

 all nice calculations founded on such measures 

 must be fallacious and uncertain, and, therefore, 

 should, for the most part, be looked on rather 

 as illustrations than strict demonstrations of the 

 points in question." The method by which 

 relit arrived at these results must render them 

 of doubtful value, the curvatures having been 

 determined by the application of brass plates 

 cut to the requisite form. The results of 

 Chossat's experiments, conducted with great 

 care, and with the assistance of the megascope, 

 are thus stated by Mr. Lloyd in his Treatise on 

 Optics : " This author has found that the cornea 

 of the eye of the ox is an ellipsoid of revolution 

 round the greater axis, this axis being inclined 

 inwards about 10. The ratio of the major 

 axis to the distance between the foci in the 

 generating ellipse he found to be 1.3 ; and this 

 agreeing very nearly with 1.337, the index of 

 refraction of the aqueous humour, it follows 

 that parallel rays will be refracted to a focus, by 

 the surface of this humour, with mathemathical 

 accuracy. The same author found likewise that 

 the two surfaces of the crystalline lens are ellip- 

 toii/s of revolution round the lesser axis ; and itis 

 somewhat remarkable thatthe axes of these sur- 

 fiu-fs tin not coincide in direction either with each 

 other, or with the axis of the cornea, these axes 

 being both inclined outwards, and containing 

 with each other, in the horizontal section in 

 which they lie, an angle of about 5." It must 

 not be forgotten that these observations apply 

 to the crystalline of the ox, not to that of m\n, 

 and also that, as Chossat himself admits, the 

 evaporation of the fluid part of the lens, or the 

 absorption or imbibition of the water in which 

 it is immersed, may materially alter the curva- 

 ture. I cannot myself believe it possible to 

 separate a fresh lens in its capsule perfectly 

 from the hyaloid membrane without injuring 

 its structure, and endangering an alteration in 

 its form. Ilaller states that Kepler considered 

 the anterior convexity to approach to a sphe- 

 roid, and the posterior to a hyperbolic cone. 

 Wintriogbam states the results of his inquiries 

 as to this matter as follows : " In order to 

 take the dimensions of the eye of an ox, I 

 placed it on a horizontal board and applied 

 three moveable silks, which were kept extended 

 by small plummets, so as to be exact tangents 

 t<> the arch of the cornea, as well at each can- 

 tlms. as at the vertex; then applying a very 

 exactly divided scale, I found that the chord of 

 the cornea was equal to 1.0.5 of an inch, the 

 MW d sine of this chord to be 0.20, and con- 

 sequently the ratlins of the contra was equal to 

 0.630215 of an inch. I then carefully took off 



the cornea, and replaced the eye as before, and 

 found, by applying one of the threads as a tan- 

 gent to the vertex of the crystalline, that the 

 distance between this and the vertex of the cor- 

 nea was 0.355 of an inch. Afterwards I took 

 the crystalline out without injuring its figure, 

 or displacing the capsula, and then applying 

 the threads to each surface of this humour, as 

 was done before to the arch of the cornea, I 

 found that the chord of the crystalline was 0.74 

 of an inch, and its versed sine, with respect to 

 the anterior surface, to be 0.189 of an inch, and 

 consequently the radius of this surface was 

 0.45665 of the same. In like manner the 

 versed sine to the same chord, with respect to 

 the posterior surface of the crystalline, I found 

 to be equal to 0.38845 of an inch. Lastly, 1 

 found the axis of the crystalline and that of the 

 whole eye from the cornea to the retina to be 

 0.574, 2.21 respectively." Whatever doubts 

 may be entertained respecting the accuracy of 

 the measurements of the lens, there can be none 

 that the form is different at different periods of 

 life, in the human subject. It also appears to 

 differ in different individuals at the same period 

 of life, and probably the curvature is not the 

 same in both eyes. In other animals the dif- 

 ference in form is most remarkable. In the 

 human fcetus, even up to the ninth month, it is 

 almost spherical. Petit states that be found 

 the anterior curvature in a foetus of seven 

 months, a portion of a sphere of three lines 

 diameter, and the posterior of two and a half, 

 and the same in a new-born infant. In an in- 

 fant eight days old, the anterior convexity was 

 a portion of a sphere of four lines, and the 

 posterior of three. All anatomists concur in 

 considering the lens to approach more to a 

 sphere at this period. In childhood the curva- 

 tures still continue much greater than in ad- 

 vanced life ; from ten to twenty probably de- 

 crease, and from that period to forty, forty-five, 

 or fifty, remain stationary, when they become 

 much less; being, according to the tables of 

 Petit, portions of spheres from seven to even 

 twelve lines in diameter, and on the posterior 

 of six or eight. Every day's observation proves 

 that the lens becomes flattened, and its curva- 

 tures diminished as persons advance in life. It 

 is seen in dissection, when extracted by opera- 

 tion, and even during life; the distance between 

 its anterior surface and the back of the iris be- 

 ing so great in some old persons, that the sha- 

 dow of the pupil may be seen upon it, while at 

 an earlier period it actually touches that part of 

 the membrane. This diminution of the curva- 

 tures of the lens commences about the age of 

 forty-five. Petit found the anterior convexity 

 varying from a sphere of about seven to twelve 

 lines diameter, and the posterior from five to 

 eight in persons from fifty to sixty-five years of 

 age. The alteration in power of adaptation, 

 and the indistinctness of vision of near objects 

 which takes place at this period, is probably to 

 be attributed to this cause, although a diminu- 

 tion of the muscular power of the iris, and con- 

 sequent inactivity of the pupil, may contribute 

 t" tin- defect. It is also to be recollected tli.n 

 the density of the lens is much increased at tlii> 

 period, and that the young person whose lens 



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