VISION. 11 



and concluded, that it was the modification of the 

 light by the eyelashes and not the compression of 

 the eye which took place. 



M. Jurine's theory is, that the cornea is com- 

 pressed and rendered more convex by the contrac- 

 tion of the iris. To render this plausible, he sup- 

 poses that there is a muscular ring- round the iris, 

 which contracts on looking at near objects, but 

 that, when looking at more distant ones, it relaxes, 

 and the cornea springs back, by its elasticity, to its 

 primary place. But, not to mention that this mus- 

 cular ring of the iris cannot be demonstrated, we 

 know that the iris is not fixed in the cornea at. all, 

 but in the inflexible white of the eye. That the 

 cornea, however, is affected in some degree, seems 

 to have been proved by Mr. Ramsden. He invented 

 an apparatus, by which the head was accurately fixed, 

 and a miscroscope adapted to observe the changes in 

 the eye whilst observing near and distant objects. 

 From very nice experiments, made with this appa- 

 ratus, it was found that the cornea moved the eight- 

 hundredth part of an inch from the nearest point of 

 distinct vision, to a distance of ninety feet*. 



Sir Everard Home has attempted to explain this 

 from the connection of the cornea with the muscles. 

 He found that they are inserted into the white coat, 

 about an eighth of an inch from the cornea ; and that 

 therefore their compression of the eyeball will force 

 the humours outwards, and thus push out the cornea. 

 Not content with this, he detached the outer layer of 

 the cornea along with the muscles, and even conceived 

 that the muscles spread over the cornea. This, as 

 Dr. Monro remarks, must destroy the supposition of 

 the cornea's being moved outwards by the muscles, as 

 their contraction would draw it inwards. 



It is another opinion, that the iris produces the 

 * Nicholson's Journ, 4to. i. 303, &c. 



