Page:Proceedings of the Royal Society of London Vol 1.djvu/128

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After a brief survey of what former physiologists, such as Bertrandi, Fourcroy, Wintringham, and Chrouet, have done in investigating the chemical history of the humours of the eye, in all which he found so considerable a disagreement, and 'so much obscurity, as to render a new analysis at least usefulif not necessary, Mr. Cheuevix proceeds to give us his own experiments. Of these, the ﬁrst series was made on the eyes of sheep, and the second on the human eye; and they of course relate to the three humours, the aqueous, the crystalline, and the vitreous.

The speciﬁc gravity of the aqueous humour of the sheep’s eye was found to be [0090, taking that of water as 10'000; and from the results of various experiments, it appears to be composed of water, albumen, gelatine, and of a muriate the basis of which was found to be soda. The speciﬁc gravity of the crystalline was equal to 11'000, and was found to consist of a smaller quantity of water than the other humoui‘s, but of much larger proportion of albumen and gela- tine; and no essential diﬁ'erence could be perceived between the vitreous humour and the aqueous, their speciﬁc gravities, as well as their several chemical properties, being the same.

From the examination of the humours of the human eye, Mr. Che.- nevix convinced himself that they are scarcely in any respect different from those of the sheep’s eye. The aqueous and vitreous humours were found to contain water, albumen, gelatine, and muriate of soda; the latter ingredient alone being wanting in the crystalline. The speciﬁc gravities of the aqueous and vitreous humours were 10053, and of the crystalline 10790.

The speciﬁc gravity of the crystalline, compared with that of the aqueous and vitreous humours, being much greater in the human eye than in that of the sheep, our author bestows some pages on an in- quiry concerning this phaenomenon; the result of which is, that as the globe of the human eye is smaller than that of the sheep, and hence the distance from the cornea to the retina much shorter, na- ture, in order to preserve the achromatic eﬁ'ect of the eye, rendered the human crystalline proportionany more dense than in other ani- mals. This illustration is conﬁrmed by the examination of the eye of an ox, where the difference between the speciﬁc gravities of the humours was as 10'088 to 10765.

In examining the eyes of birds, it was found, that, different from those of quadrupeds, the cornea, or the anterior part of the eye, is a portion of a larger sphere than the sclerotim, or posterior part of the ball. It is hence obvious that, in order to produce a proper refraction, it is necessary that the densities of the humours be essentially different. Accordingly, it was found that the speciﬁc gravity of the vitreous humour was 11'210; while that of the crystalline was no more than 10392. “hence it appears, that the densitics are here