68 
Dr. Young’s Lecture 
Benjamin Clerk’s eye. It is also a fact of no small impor- 
tance, that Sir Henry Englefield was much astonished, as 
well as the other observers, at the accuracy with which the 
man’s eye was adjusted to the same distance, in the repeated 
trials that were made with it, * This circumstance alone makes 
it highly probable, that its perfect vision was confined within 
very narrow limits. 
Hitherto I have endeavoured to shew the inconveniences 
attending other suppositions, and to remove the objections to 
the opinion of an internal change of the figure of the lens. 
I shall now state two experiments, which, in the first place, come 
very near to a mathematical demonstration of the existence of 
such a change, and, in the second, explain in great measure its 
origin, and the manner in which it is effected. 
I have already described the appearances of the imperfect 
image of a minute point at different distances from the eye, in 
a state of relaxation. For the present purpose, I will only 
repeat, that if the point is beyond the furthest focal distance of 
the eye, it assumes that appearance which is generally described 
by the name of a star, the central part being considerably the 
brightest. (Plate VI. Fig. 3 6 — 39.) But, when the focal dis- 
tance of the eye is shortened, the imperfect image is of course 
enlarged ; and, besides this necessary consequence, the light is 
also very differently distributed ; the central part becomes faint, 
and the margin strongly illuminated, so as to have almost 
the appearance of an oval ring. (Fig. 41.) If I apply the 
slider of the optometer, the shadows of the slits, while the eye 
is relaxed, are perfectly straight, dividing the oval either way 
into parallel segments : (Fig 42, 44.) but, when the accom- 
* Phil. Trans, for 1795. p. 8. 
