Page:Proceedings of the Royal Society of London Vol 1.djvu/464

Rh subject to whatever uncertainty remains with regard to astronomical refraction.

As the Astronomer Royal has not been in possession of the instrument a sufficient length of time for deducing the real quantity of refraction from his own observations, he has hitherto employed those of Dr. Bradley, which have been many years in use at the Royal Observatory; but he observes, that any alteration which may be hereafter found necessary, may easily be made as correction to the above observations.

The author states, in the ﬁrst place, that he has rarely observed short-sightedness in children under ten years of age, and that he considers it as commencing generally between that period and eighteen; that it at ﬁrst occasions so little inconvenience, that it is not noticed by those who have not access to concave glasses, and consequently is very frequently overcome by the natural efforts of the eye. Persons, on the contrary, in the higher ranks of society, who have it more in their power to indulge a slight weakness, by having recourse to short—sighted glasses, soon conﬁrm the imperfection, and in many instances even render it" worse, by employing glasses deeper than are necessary.

For the purpose of instituting a comparison between the proportional prevalence of this defect in different classes of society, Mr. Ware made inquiry in the three regiments of Foot Guards, containing nearly 10,000 men, and also in the two Universities, Oxford and Cambridge. In the Guards short-sightedness among the privates is scarcely known; and not more than half a dozen recruits are said to have been rejected for this imperfection in the course of twenty years. In the Universities, on the contrary, the numbers are so considerable, that in one of the colleges in Oxford, it is said that of 127 persons, so many as thirty-two have used either a hand-glass or spectacles. It is thus proved that short-sightedness is corrected in one class of persons, and encouraged in the other; and it is evident that even in those who absolutely require glasses, it may be increased by using such as are deeper than are really necessary, or counteracted by employing the lowest with which the eye can see with comfort.

It is observed, however, that extreme short-sightedness sometimes occurs in children from visible imperfection in the form of the cornea, and sometimes arises as an accidental and temporary consequence of general debility, and is then removable by chalybeate medicines and bracing applications.

Dr. Porterﬁeld has observed, that the pupils of short-sighted persons are in general more dilated than those of others; but Mr. Ware does not admit this to be generally the case; neither does he admit