210 
Dr. Hosack's Observations on Vision. 
appears too obvious to need further illustration. I shall there- 
fore 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 six feet, the picture of 
the object falls exactly upon the retina ; I now direct my at- 
tention to an object at the distance of six inches, as nearly as 
possible in the same line ; although the rays from the first 
object still fall upon my eye, while viewing the second, it does 
not form a distinct picture on the retina, although at the same 
distance as before, which shews that the eye has undergone 
some change ; for while I was viewing the first object I did 
not see thesecond distinctly, although in the same line : and 
now vice versa, I see the second distinctly, and not the first ; 
the rays from the first, therefore, as they still fall upon the 
eye, must either meet before or behind the retina; but we 
have shewn that the rays from the more distant object con- 
vene sooner than those from the less distant object, therefore 
the picture of the object at six feet falls before, while the other 
forms a distinct image upon the retina ; but as my eye is stiU 
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, agree- 
able to the principle page 198. 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 shewn 
that the lens, which is the greatest refracting medium, has no 
power of changing itself. Let us next inquire, if the external 
muscles, the only remaining power the eye possesses, are ca- 
pable of producing those changes. With respect to the 
