VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 465 



of an action, or exertion, was very evident to every gentleman concerned in these 

 experiments. In changing the focus of our eyes, we were much astonished, 

 particularly Sir Henry Englefield, at the exertion required to adjust the eye to 

 the near distances, and the facility with which it was adapted to distant ones ; 

 the first was a strain on the eye, the 2d appeared a relief to it. When the eye 

 was intent on the near object, it required the attention to be constantly kept up, 

 or the object became indistinct ; and if we looked at it beyond a certain time, the 

 eye was so much fatigued as to lose it at intervals. This corresponds with other 

 muscular actions, for whenever muscles are kept long in one state they begin to 

 vibrate involuntarily. 



These circumstances explain what may be called a coup d'oeil, or the distinct- 

 ness with which an object is seen when the eye is first fixed on it. This arises 

 from the nice adjustment produced by the muscles when first thrown into action, 

 which they cannot keep up, being unable to remain long in the same state ; nor 

 can they, after having been used for, any time, return to this adjustment with 

 the same exactness. 



The change that takes place in the eye at an advanced period of life, by which 

 it loses its adjustment to very near, and very distant objects, does not arise from 

 any defect in the muscles, as might at first be imagined, since that would not 

 account for the eye being unable to see with parallel rays ; nor is there any ob- 

 vious reason why these muscles should lose their powers, while others, which are 

 not apparently so strong, if we may judge by their effects, retain their full action 

 long after the eye has undergone this change. This defect in the eye, I am led 

 to believe, is brought on by the cornea losing its elasticity as we advance in life, 

 neither contracting nor being elongated to its usual extent, but remaining in a 

 middle state. That elastic substances in the body do undergo such a change, 

 may be well illustrated in the vascular system. The aorta is composed almost 

 entirely of elastic substance, and there is probably no part of the body, at an 

 advanced age, which is so often found to have lost its natural action ; it appears 

 to undergo a change from age alone, becoming inelastic, and then taking on dis- 

 eases of different kinds, as being ossified, or becoming aneurismal ; but in nei- 

 ther of these diseases is it found to be contracted, though often the reverse, 

 and when disease has not supervened, the artery more commonly remains in the 

 middle state. 



The cornea, having similar properties, must be liable to a similar change; but 

 its action being less constant, and the power which is to resist being weaker, 

 the change will be probably more gradual and less in degree, but sufiicient to 

 account for the alteration we find in the focus of the eyes of old people. There 

 are many other circumstances respecting vision, and many which occur in disease, 

 that may be explained by a knowledge of these faots ; but as this lecture is only 



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