Volume 182303

The works of William Paley / [William Paley].

William Paley

Date:: 1823

Credit: The works of William Paley / [William Paley]. Source: Wellcome Collection.

35/388 page 15

$do it. The point of concourse behind the lens must fall eritically 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 swhen 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 telescope; or by adjusting the distance of his _ glasses with his hand or hisscrew : but how is it to be ma- -Maged 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 laws which govern the refrac- tien 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 suffi- _ cient for the purpose, is so minute as to elude ordinary -observation. Some very late discoveries, deduced from a laborious and most accurate inspection of the struc- ture and operation of the organ, seem at length to have ascertained the mechanica! alteration which the parts of the eye undergo. It is found, that by the action of -tertain muscles, 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 pro- duced in it at the same time, all severally contributing ‘to the adjustment required. The cornea, or outermost ,eoat of the eye, is rendered more round and prominent ; othe crystalline lens underneath is pushed forward ; and hAhe-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 ob- ject is placed at a distance. Can any thing be more de- » Gisive of contrivance than this is? The most secret laws --0f optics must have been known to the author of a » Structure eadowed with such a capacity of change. It$

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