18 APPLICATION OF THE ARGUMENT. 



lie near at hand, within a few inches, we will suppose, oi 

 the eye, and of objects which were placed at a considerable 

 distance from it, that, for example, of as many furlongs, 

 (I speak in both cases of the distance at which distinct 

 vision can be exercised.) Now, this, according to the 

 principles of optics, that is, according to the laws by which 

 the transmission of light is regulated, (and these laws are 

 fixed,) could not be done, without the organ itself under- 

 going an alteration, and receiving an adjustment, that 

 might correspond witn the exigency of the case, that is to 

 say, with the different inclination to one another under 

 which the rays of light reached it. Rays issuing from points 

 placed at a small distance from the eye, and which conse- 

 quently nmst enter the eye in a spreading or diverging 

 order, cannot, by the same optical instrument in the same 

 state, be brought to a point, i. e. be made to form an image, 

 in the same place with rays proceeding from objects situat- 

 ed at a much greater distance, and which rays arrive at 

 the eye in directions nearly, and, physically speaking,, 

 parallel. It requires a rounder lens to do it. The point 

 of concourse behind the lens must fall critically upon the 

 retina, or the vision is confused ; yet this point, by the im- 

 mutable properties of light, is carried further back, when 

 the rays proceed from a near object, than when they are 

 sent from one that is remote. A person, who was using 

 an optical instrument, would manage this matter by chang- 

 ing, as the occasion required, his lens or his telescope ; 

 or by adjusting the distance of his glasses with his hand 

 or his screw ; but how is it to be managed in*the eye? 

 What the alteration was, or in what part of the eye it took 

 place, by what means it was effected (for, if the known 

 laws which govern the refraction of light be maintained, 

 some alteration in the state of the organ there must be,) 

 had long formed a subject of inquiry and conjecture. 

 The change, though suiRcient for the purpose, is so minute 

 as to elude ordinary observation. Some very late discove- 

 ries, deduced from a laborious and most accurate inspection 

 of the structure and operation of theorgaU; seem at length 

 to have ascertained the mechanical alteration which the 

 parts of the eye undergo. It is found, that by the action of 

 certain muscles, [PI. II. fig. 7.] called the straight muscles, 

 and which action is the most advantageous that could be 

 imagined for the purpose, — it is found, I say, that whenever 

 the eye is directed to a near object, three changes are produc- 

 ed in it at the same time, all severally contributing to the ad-^ 



