16 
APPLICATION OF THE ARGUMENT. 
made from different materials, the effects of the different 
humours through which the rays of light pass before they 
«each the bottom of the eye. Could this be in the eye 
..without purpose, which suggested to the optician the only 
effectual means of attaining that purpose ? * 
But farther; there are other points, not so much perhaps 
of strict resemblance between the two, as of superiority of 
the eye over the telescope, which being found in the laws 
that regulate both, may furnish topics of fair and just com¬ 
parison. Two things were wanted, to the eye, which 
were not wanted (at least in the same degree) to the teles¬ 
cope: and these were the adaptation of the organ, first, 
to different degrees of light; and, secondly, to the vast 
diversity of distance at which objects are viewed by the 
naked eye, viz. from a few inches to as many miles. These 
difficulties present not themselves to the maker of the 
telescope. He wants all the light he can get; and he 
never directs his instrument to objects near at hand. In 
the eye, both these cases were to be provided for; and for 
the purpose of providing for them a subtile and appropriate 
mechanism is introduced:— 
* “ It does not appear that the hint of this discovery was taken by 
Mr. Dollond from the structure of the eye, a3 supposed by our author, 
but was obtained in a different manner. This circumstance does not 
however lessen the force of the reasoning. The principle thus applied 
in the construction of achromatic telescopes, has been since carried still 
farther, and in its new application, illustrates more strongly, if possible, 
the point so well insisted on by Dr. Paley, namely, the resemblance be¬ 
tween the eye and our optical instruments. In the best achromatic tele¬ 
scopes, composed of the different kinds of glass, according to the discov¬ 
ery of Mr. Dollond, white or luminous objects are not shown perfectly free 
from color, their edges being tinged on one side with a claret colored, 
and on the other with a greenish fringe. This remaining imperfection 
has been got rid of by the combination of solid and fluid lenses in the 
object and eye-glasses of telescopes. For this beautiful discovery science 
is indebted to Dr. Blair of Edinburgh, who found that by placing a con¬ 
cave lens of muriatic acid with a metallic solution, between two convex 
lenses of glass, a combined lens was formed which refracted rays with 
perfect regularity and equality. A lens like this has been used with 
great advantage. The most important point is, however, to consider 
this improvement in its application to the argument, and it will be seen 
how much nearer this construction brings the telescope to the eye. In 
Dollond’s telescope there is a combination of solid lenses of different 
substances.—In Blair’s, a combination of fluid and solid ; which is ex¬ 
actly the case in the human eye. The only difference is, that in the eye 
there is a solid lens between two fluid ones ; and in the telescope a fluid 
between two solid. The combination is closely similar, and the final 
cause in both probably the same, namely, to correct the unequal refrac¬ 
tion of light.”—See Edinburgh Journal of Science y No. viii. p. 212 : 
and Library of Useful Knowledge , No. 1 & 12. [Ed. 
