APPLICATION OF THE ARGUMENT. 
17 
I. In order to exclude excess of light, when it is ex 
cessive, and to render objects visible under obscurer degrees 
of it, when no more can be had, the hole or aperture in 
the eye, through which the light enters, is so formed, as to 
contract or dilate itself for the purpose of admitting a great¬ 
er or less number of rays at the same time. The cham¬ 
ber of the eye is a camera obscura,* which, when the light 
is too small, can enlarge its opening; when too strong, 
can again contract it; and that without any other assist¬ 
ance than that of its own exquisite machinery. It is far¬ 
ther also, in the human subject, to be observed, that this 
hole in the eye, which we call the pupil, under all its dif¬ 
ferent dimensions, retains its exact circular shape. This 
is a structure extremely artificial. Let an artist only try 
to execute the same; he will find that his threads and 
strings must be disposed with great consideration and con¬ 
trivance to make a circle, which shall continually change 
its diameter, yet preserve its form. This is done in the 
eye by an application of fibres, i. e. of strings, similar, in 
their position and action, to what an artist would and must 
employ, if he had the same piece of workmanship to per¬ 
form. [Plate II. Fig. 5 & 6.] f 
* As the rays of light flowing from all the points of an object through 
the pupil of the eye, by the refraction of the lens and humours of the 
eye, form an exact representation at the bottom of the eye on the retina ; 
so the camera obscura, by means of a lens refracting the rays, exhibits a 
picture of the scene before it on the opposite wall.— Paxton. 
t Some eminent anatomists have doubted the muscularity of the iris, 
and have given very different explanations of its motions, attributing the 
contraction and dilatation either to the varied impulse of the blood in its 
vessels, or to its own vita propria. The enlightened physiologist Magen- 
die affirms, that the latest researches upon the anatomy of the iris proves 
its muscular structure, and that it is composed of two layers of fibres, the 
external, Plate II. (Fig. 5.) radiated , which dilate the pupil, the other 
(Fig. 6.). circular, which contract the pupil. The external circular 
fibres appear to be supported by a species of ring, which each of the ra¬ 
diated fibres contribute to form, and in which they slide during the alter¬ 
nate contractions and relaxations of the pupil.— Paxton. 
There is a curious circumstance in the way in which light produces the 
contraction of the opening of the iris, which strengthens very much the ar¬ 
gument derived from design manifested in its structure and adaptation to its 
purpose. The object of the iris, it is to be observed, has reference to the 
quantity of light to be admitted upon the retina or expansion of f he optic 
nerve. It is the state of the retina then which regulates the motions of the 
iris, and it is the action of the light on the retina which causes those mo¬ 
tions and not its action upon the iris itself. This has been shown by a very 
delicate experiment. If a'ray of light be accurately thrown in such a 
direction, that it shall fall upon the circle of the iris itself, and not pass 
through its aperture, no contraction of the aperture takes place; but if it 
