410 PHILOSOPHICAL TRANSACTIONS. [aNNO 1794. 



and belly, making a compression on the viscera, as in the discharge of urine, 

 foeces, &c. But to question this fact would be to question the influence cf the 

 will in any one of the almost infinite variety of motions in the human body. 



I presume therefore it will be admitted that we have the same power over these 

 muscles of the eye as of others, and I believe we are no less sensible of their 

 combined action ; for example, after viewing an object at the distance of half a 

 mile, if we direct our attention to an object but 10 feet distance, every person 

 must be sensible of some exertion ; and if our attention be continued but for a 

 short time, a degree of uneasiness and even pain in the ball of the eye is expe- 

 rienced ; if again we view an object within the focal distance, i. e. within 6 or 7 

 inches^, such is the intensity of the pain that the exertion can be continued but 

 a very short time, and we again relieve it by looking at the more distant objects ; 

 this I believe must be the experience of every person whose eyes are in the na- 

 tural and healthy state, and accordingly has been observed by almost every writer 

 on optics. But the power of this combination, even from analogy, appears too 

 obvious to need further illustration. I shall therefore next endeavour to point 

 out their precise action. 



Supposing the eye in its horizontal natural position ; I see an object distinctly 

 at the distance of 6 feet, the picture of the object falls exactly on the retina ; I 

 now direct my attention to an object at the distance of 6 inches, as nearly as pos- 

 sible in the same line ; though the rays from the first object still fall on my eye, 

 while viewing the 2d, it does not form a distinct picture on the retina, though at 

 the same distance as before, which shows that the eye has undergone some 

 change ; for while I was viewing the first object I did not see the 2d distinctly, 

 though in the same line : and now, vice versa, I see the 2d distinctly, and not 

 the first ; the rays from the first therefore, as they still fall on the eye, must 

 either meet before or behind the retina; but we have shown that the rays from 

 the more distant object convene sooner than those from the less distant object, 

 therefore the picture of the object at 6 feet falls before, while the other forms 

 a distinct image on the retina ; but as my eye is still in the same place as at first, 

 the retina has by some means or other been removed to a greater distance from 

 the fore part of the eye to receive the picture of the nearer object, agreeable to 

 the principle before-mentioned. From which it is evident, that to see the less 

 distant object, either the retina should be removed to a greater distance or the 

 refracting power of the media should be increased : but I hope we have shown 

 that the lens, which is the greatest refracting medium, has no power of chang- 

 ing itself. 



Let us next inquire, if the external muscles, the only remaining power the eye 

 possesses, are capable of producing those changes. With respect to the anterior 

 part of the eye, we have seen the situation of those muscles ; the recti strong 



