Page:Proceedings of the Royal Society of London Vol 1.djvu/465

Rh the common opinion to be well founded, that the magnitude of the pupil varies according to the distance of the object viewed. He names, however, one extraordinary instance of a lady, Whose pupil contracts only when she views objects at the distance of nine inches, but at other times is dilated very nearly to the full extent of the rim of the cornea.

Mr. Ware has also made experiments similar to those of Dr. Wells, on the alteration of the power of the eye during that dilatation of the pupil which is produced by the external application of belladonna. Those of the author were attended with the same result of lengthening the focus of either eye to which the belladonna was applied, without affecting the customary range of vision in the other eye.

The author observes, that short-sightedness does not depend on the greater or less concavity of the cornea alone; since its distance from the retina, and the convexity of the crystalline also, must be taken into the account.

It has sometimes been observed, that old persons have in a short time recovered the perfect sight of younger persons; and this has been explained by Dr. Porterﬁeld, by a supposition of the absorption of fat from the bottom of the orbit, allowing the axis of the eye to become more elongated : but Mr. Ware thinks it more likely to have arisen from absorption of the vitreous humour, in consequence of which the sclerotica would be pressed inwards, and that then the axis of the eye would be elongated. 7

Mr. Ware observes, that persons in general who use glasses, pos- sess the power of seeing both near and distant objects with the same glass, but that this is not the case with those who have been couched, who always require a different glass to enable them to see distant objects; proving thereby that the adapting power of the eye depends on the presence of the crystalline. In consequence of this defect, such persons judge very imperfectly of distances. .

In comparing the range of adaptation possessed by short-sighted persons with that of others, the author conceives the range of the former to be much less; his estimate being made by the number of inches through which the range of distinct vision extends.

The author, being himself very short-sighted, remarks, that the change that has taken place in his own eyes by age, does not accord with the observation generally made, that short-sighted persons become less so as they advance in life. In his eyes the shortest distance of distinct vision remains nearly where it was; but the power of discerning distant objects is so far lessened, that for this purpose he requires a glass one degree deeper than that which he commonly employs, and with which he formerly used to distinguish distant as well as near objects; and as he is acquainted with other instances in which a correspondent change has taken place, he is of opinion that such changes are by no means unfrequent. However, in two of those here enumerated, this change was produced by evident disease; and in one of them it was only temporary. A third instance mentioned of an eye becoming less long-sighted, is occasioned by unusual efforts