662 PHILOSOPHICAL TRANSACTIONS. [aNNO 1796. 



observation, than it was possible to do it, had the object been seen with the ne- 

 cessary degree of distinctness. 



Finding from these experiments, that the change in the convexity of the 

 cornea was not to be seen distinctly in the micmmeter, it became an object to 

 ascertain the degree of change which could in this way be distinctly determined. 

 For this purpose 2 mirrors were ground, and prepared in the same way as those 

 used in the preceding experiment ; their radii were exactly ascertained by mea- 

 suring the tools in which thty were finished off; the one was ^V of an inch 

 focus, the other -rVW ' the difference between the size of the images reflected 

 from their surfece was just visible in the micrometer; and from their remaining 

 fixed, the experiment could be made with every advantage ; but it did not ap- 

 pear 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 on 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 experiment which any one may make on 

 himself: let him take a glass spirit level, and rest one end of it on a table, sup- 

 porting 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 -i- 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 ^{-^ part of an inch. 

 This change in tlie 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 lens, as considerably to lessen their effect in fitting the eye for seeing 

 near objects, and make this small increase of convexity inadequate to such an 

 effect. Finding this to be the case, it became necessary to examine the eye with 

 attention, to see in what way the full effect was most likely to be produced. 

 For this purpose the following experiments were made on the human eye, to de- 



