18 
APPLICATION OF THE ARGUMENT. 
II. The second difficulty which has been stated, was the 
suiting of the same organ to the perception of objects that 
lie near at hand, within a few inches, we will suppose, of 
the eye, and of objects which are 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 with 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 must 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 other things remaining the same, 
this point, by the immutable properties of light, is carried 
farther 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 changing, as the occasion required, his lens or his tele¬ 
scopes; 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, or by what means it was effected, (for if the known 
be so thrown as to pass through the aperture, and fall upon the retina 
without touching the iris at all, still a contraction of the iris immediately 
takes place. So that light upon the iris alone occasions no contraction, 
although it is the part which really contracts when the same light falls 
upon a distant part. The design here is too obvious to need being en¬ 
larged upon. How could the iris acquire the power of contracting when 
light fills on another membrane, for the protection of that membrane ? 
although it does not contract when the light falls upon itself alone ?— [Ed. 
* The focus of the refracted rays must fall exactly on the retina, so 
that the point of vision may be neither produced beyond it, nor shorten¬ 
ed so as not to reach it. The latter defect exists in short-sighted per¬ 
sons, from too great convexity of the cornea or lens. The former is the 
defect of long-sighted persons, in whom there is an opposite conforma¬ 
tion of those parts.— Paxton. 
