4 
Mr. Home's Lecture 
appearance. The effect produced on the micrometer, there- 
fore, did not seem to depend upon external circumstances, but 
to arise from a change in the cornea ; it was, however, too 
small to admit of any conclusions being drawn from it. 
The same experiment was made upon several young per- 
sons ; but we found it necessary, that whoever was the subject 
of the experiment should understand perfectly what was meant 
to be done, otherwise the conclusions could not be depended 
on ; for if the eye does not see the near object with a very 
defined outline, it is not accurately adjusted to it ; and the 
length of time they kept their eye upon the near object without 
making any complaint of being fatigued, was greater, we knew, 
from our own observation, than it was possible to do it, had the 
object been seen with the necessary degree of distinctness. 
I have to regret that Sir Henry Englefield, who took a 
part in the former experiments, and whose assistance in mak- 
ing these would have been of material advantage, was unable 
to remain in town. 
Finding from these experiments, that the change in the 
convexity of the cornea was not to be seen distinctly in the 
micrometer, it became an object to ascertain the degree of 
change which could in this way be distinctly determined. 
For this purpose two mirrors were ground, and prepared in 
the same way as those used in the preceding experiment ; 
their radii were exactly ascertained by measuring the tools in 
which they were finished off; the one was of an inch 
focus, the other ; the difference between the size of the 
images reflected from their surface was just visible in the mi- 
crometer ; and from their remaining fixed, the experiment 
could be made with every advantage ; but it did not appear 
