on Muscular Motion. 
5 
probable that the same difference would have been visible had 
the mirror not been perfectly at rest. A smaller change could 
not therefore be detected in the eye ; and when we consider 
the disadvantages under which an experiment of this nature 
must be made upon the human eye, from the unsteadiness of 
that organ, the short time it remains adjusted (a part of which 
is lost in bringing it within the focus of the microscope), and 
also from the motions of the head ; it is not unreasonable to 
suppose that a change might take place in the cornea, to the 
same extent, without being distinctly seen. 
To give an idea of the short time that a part can remain 
nicely adjusted by muscular action, I shall point out an expe- 
riment which any one may make upon himself : let him take 
a glass spirit level, and rest one end of it on a table, supporting 
the other with his hand, and endeavour to keep the air bubble 
in the middle ; if the hand is very steady the bubble may be 
kept nearly in its place, but not exactly so, it will undulate, 
its motion corresponding with the actions of the muscles ; 
making up for want of steadiness by short motions in contrary 
directions. 
From these experiments the change in the curvature of the 
cornea could not be more than T ~ part of an inch, as any 
greater quantity would probably have been distinctly seen in 
the micrometer ; this, however, is still more than was ascer- 
tained by our former experiments, which made it to exceed 
P art °f an inch- 
This change in the cornea, on the first view of the subject, 
appeared sufficient to account for the adjustment of the eye, and 
when the lens is removed it probably may be sufficient ; but the 
refractions at the cornea are so much changed by those at the 
