2 
Mr. Home's Lecture 
be made to it, and to put it to the test of such experiments 
as appeared likely to refute or confirm our former observations. 
It readily suggested itself, that if the convexity of the cornea 
was increased to a certain degree, it could be measured by 
means of an image reflected from its surface, and viewed in an 
achromatic microscope with a divided eye-glass micrometer. 
To ascertain whether the quantity of increase of the con- 
vexity of the cornea, in the adjustment of the eye, could in 
this way be ascertained, the following experiments were con- 
trived, and made by Mr. Ramsden. 
Our former experiments had sufficiently proved the unstea- 
diness of the human eye ; the first trials on the present occa- 
sion were therefore made upon convex mirrors, as these arti- 
ficial corneas could be more readily managed, and such previous 
experiments would enable us to apply the same instruments 
with more facility to the eye itself. 
Two convex mirrors, one of an inch focus, the other 
t 5 q, had their flat surfaces made rough, and blacked, to pre- 
vent an image being seen from both surfaces, which was 
found to be the case when this precaution was omitted. One 
of these mirrors was stuck upon a piece of wood directly oppo- 
site to a window, at twelve feet distance from it ; a board, 
three feet long, and six inches broad, was placed perpendicu- 
larly against the sash of the window, and its image reflected 
from the mirror upon the object-glass of an achromatic mi- 
croscope, with a divided eye-glass micrometer. 
The two images were separated by means of the divided 
eye-glass, till their surface of contact, which appears like a 
black line, was rendered as small as possible. When this ef- 
fect was produced on the images from the mirror of of an 
