APPLICATION OF THE ARGUMENT. 
15 
matter how) to the sense of sight, and to the exercise of 
that sense, the apparatus by which it is formed is con¬ 
structed and put together, not only with infinitely more art, 
but upon the selfsame principles of art, as in the telescope 
or the camera obscura. The perception arising from the 
image may be laid out of the question; for the production 
of the image, these are instruments of the same kind. 
The end is the same; the means are the same. The pur¬ 
pose in both is alike, the contrivance for accomplishing 
that purpose is in both alike.* The lenses of the telescope, 
[Plate II. fig. 3, 4.] and the humours of the eye, bear a 
complete resemblance to one another, in their figure, their 
position, and in their power over the rays of light, viz. in 
bringing each pencil to a point at the right distance from 
the lens; namely, in the eye, at the exact place where the 
membrane is spread to receive it. How is it possible, un¬ 
der circumstances of such close affinity, and under the 
operation of equal evidence, to exclude contrivance from 
the one, yet to acknowledge the proof of contrivance hav¬ 
ing been employed, as the plainest and clearest of all pro¬ 
positions, in the other? 
The resemblance between the two cases is still more ac¬ 
curate, and obtains in more points than we have yet repre¬ 
sented, or than we are, on the first view of the subject, 
aware of. In dioptric telescopes there is an imperfection 
of this nature. Pencils of light, in passing through glass 
lenses, are separated into different colors, thereby tinging 
the object, especially the edges of it, as if it were viewed 
through a prism. To correct this inconvenience had been 
long a desideratum in the art. At last it came into the 
mind of a sagacious optician, to inquire how this matter 
was managed in the eye; in which there was exactly the 
same difficulty to contend with as in the telescope. His 
observation taught him, that, in the eye, the evil was cur¬ 
ed by combining lenses composed of different substances, 
i. e. of substances which possessed different refracting 
powers. Our artist borrowed thence his hint; and pro¬ 
duced a correction of the defect by imitating, in glasses 
* The comparison with the lens of the telescope is not perfectly exact, 
for the crystalline lens is a substance composed of concentric layers, of 
unequal density, the hardness of which increases from the surface to the 
centre; and hence possesses a more refractive power than any artificial 
lens. Mr. Ramsden supposes that this texture tends to correct the aber¬ 
ration occasioned by the spherical form of the cornea, and the focus of 
each oblique pencil of rays falls accurately on the concave surface of 
the retina.— Paxton. 
