APPLICATION OF THE ARGUMENT. 
19 
\vs 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 sufficient 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 the organ, 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¬ 
justment required. The cornea, or outermost coat of the 
eye, is rendered more round and prominent; the crystalline 
lens underneath is pushed forwards; and the axis of 
vision, as the depth of the eye is called, is elongated. 
These changes in the eye vary its power over the rays of 
light in such a manner and degree as to produce exactly 
the effect which is wanted, viz. the formation of an image 
upon the retina, whether the rays come to the eye in a 
state of divergency, which is the case when the object is 
near to the eye, or come parallel to one another, which is 
the case when the object is placed at a distance. Can any¬ 
thing be more decisive of contrivance than this is? The 
most secret laws of optics must have been known to the 
author of a structure endowed with such a capacity of 
change. It is as though an optician, when he had a 
nearer object to view, should rectify his instrument by 
putting in another glass, at the same time drawing out 
also his tube to a different length. 
Observe a new-born child first lifting up its eyelids. 
What does the opening of the curtain discover? The an¬ 
terior part of two pellucid globes, which, when they come 
to be examined, are found to be constructed upon strict op¬ 
tical principles ; the selfsame principles upon which we 
ourselves construct optical instruments. We find them 
perfect for the purpose of forming an image by refraction; 
composed of parts executing different offices ; one part 
having fulfilled its office upon the pencil of light, deliver¬ 
ing it over to the action of another part; that to a third, 
and so onward; the progressive action depending for its 
success upon the nicest and minutest adjustment of the 
parts concerned; yet these parts so in fact adjusted, as to 
