on Muscular Motion . 
inch focus, that mirror was removed, and the other put in its 
place ; the contact of the two images, which before appeared 
like a line, had now acquired considerable breadth ; corre- 
sponding exactly to the difference between the convexities of 
the mirrors. 
Having in this way made trial of the instruments, and ar- 
ranged all the necessary circumstances, the head of a person 
was so placed as to bring the eye into the same situation as the 
mirror, and made steady by the apparatus described in our 
former experiments. Under these circumstances the image re- 
flected from the cornea was measured by the micrometer. 
Mr. Ramsden made an experiment with this instrument 
upon my eye. In the first trials, when the eye was fresh, 
there was a perceptible change in the micrometer, but ex- 
tremely small ; this was not, however, seen afterwards, and 
the eye very soon became so much fatigued that it was neces- 
sary to desist. He found that every time the eye adapted it- 
self to different distances, it was necessary to move the object- 
glass of the microscope further from, or nearer, to, the cornea. 
This experiment was repeated on four different days ; and 
in each experiment, on the first trial, the result was a change 
in the micrometer, but in all the subsequent trials it could not 
be detected. We were induced to conclude, that the effect on 
the micrometer might arise from the head being moved for- 
wards, as we found, in making experiments with the mirror, 
that this effect could be produced by such motion ; but had 
it arisen from that cause, it should more frequently have oc- 
curred, and rather after the head and eye were tired, than on 
the first trials. It was supposed to arise from the action of the 
muscles of the head, but that should have produced a contrary 
B 2 
