63 



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 hert 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 crystalline lens does not de- 

 prive the eye of the power of seeing distinctly at different distances. 



An additional case is here mentioned of a man who had a cataract 

 extracted from each of his eyes, and yet preserved a considerable 

 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- 

 periments 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 8'3 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 

 difference of the instruments. 



The Bakerian Lecture. On the Theory of Light and Colours. By 

 Thomas Young, M.D. F.R.S. Professor of Natural Philosophy in 

 the Royal Institution. Read November, 12, 1801. [Phil. Trans. 

 1802,^.12.] 



Although the mode, much practised by the ancients, of accounting 

 for a variety of phenomena 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 



