Page:Proceedings of the Royal Society of London Vol 1.djvu/83

Rh gratitude to our late ingenious brother, Mr. Ramsden, to Whom he says he is chieﬂy indebted, not only for the information which was necessary to enable him to prosecute his investigations upon the sub- ject of vision, but also the zeal which inﬂuenced his early exertions in the philosophical career.

The opinion here alluded to was brought forward in Mr. Home’s Lecture for the year 1794, and Was founded upon experiments which seemed to prove that the removal of the mystulline lens does not de- prive the eye of the power of seeing distinctly at diferent distances.

An additional case is here mentioned of a man who had a cataract extracted from each of his eyes. and yet preserved aconsiderable range of vision.

In the Bakerian Lecture of last year, Dr. Young, having entered minutely into the inquiry, thought himself authorized to doubt the above inference; and in order to insure the accuracy of the ex- perimeuts he intended to make on the subject, he constructed an optometer upon the principle of that of Dr. Porterﬁeld, by which he could ascertain the different focal lengths, and hence the power of adjustment of every eye. The result of his experiments was, that eyes deprived of the crystalline lens have lost their power of adjust- ment.

This difference of results induced Mr. Home to reconsider the sub- ject, and having sent for the man from whose eyes he had last ex. tracted the cataracts, he repeated the experiments with Dr. Young’s optometer, somewhat simpliﬁed by leaving out the lens Which was placed before the eye. With this instrument that man Was un- questionably found to have distinct vision at different distances, the nearest focus being at only 83 inches, and the furthest at 13'3 inches, while with Dr. Young's Optometer he could never observe any dif- ference Whatever.

Besides this individual, others, Whose eyes had never been dis- ordered, tried the effects of both optometers; and it should seem, from the various impressions produced upon them, that the contra- diction in the above results depends chieﬂy, if not entirely. on the diﬁ'erence of the instruments.

Although the mode, much practised by the ancients, of accounting for a variety of phaanomena by a preconceived hypothesis, be, if not wholly exploded, at least greatly discountenanced by modern phi- losophers; yet it must be owned that when a number of facts have been collected and duly ascertained, it cannot but be conducive to the extension of knowledge, to arrange them under certain heads, and, if possible, to ascribe them to some general cause; and that with men who are candid and not over-tenacious, even an error in