VOL. LXXXVI.] PHILOSOPHICAL TRANSACTIONS. 66i 



dow, at 1 2 feet distance from it ; a board 3 feet long, and 6 inches broad, was 

 placed perpendicularly against the sash of the window, and its image reflected 

 from the mirror on the object-glass of an achromatic microscope, with a divided 

 eye-glass micrometer. The 2 images were separated by means of the divided 

 eye-glass, till their surface of contact, which appears like a black line, was ren- 

 dered as small as possible. When this effect was produced on the images from 

 the mirror of -^ of an inch focus, that mirror was removed, and the other put 

 in its place ; the contact of the 2 images, which before appeared like a line, had 

 now acquired considerable breadth ; corresponding exactly to the difference be- 

 tween the convexities of the mirrors. 



Having in this way made trial of the instruments, and arranged all the ne- 

 cessary 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 reflected from 

 the cornea was measured by the micrometer. Mr, Ramsden made an experi- 

 ment with this instrument on my eye. In the first trials, when the eye was 

 fresh, there was a perceptible change in the micrometer, bul extremely small ; 

 this was not however seen afterwards, and the eye very soon became so much 

 fatigued that it was necessary to desist. He found that every time the eye 

 adapted itself to different distances, it was necessary to move the object-glass of 

 the microscope farther from, or nearer to, the cornea. 



This experiment was repeated on 4 different days ; and in each experiment, 

 on the first trial, the result was a change in the micrometer, but in all the sub- 

 sequent trials it could not be detected. We were induced to conclude, that the 

 effect on the micrometer might arise from the head being moved forwards, 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 occurred, 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 appearance. The effect pro- 

 duced on the micrometer therefore did not seem to depend on external circum- 

 stances, 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 on several young persons ; 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 on the near object without 

 making any complaint of being fatigued, was greater, we knew from our own 



