THE AHaUMENT APPLIED. 25 



tance, anJ which rays arrive at the eye in directions nearly, 

 (and physically speaking) parallel. It requires a rounder 

 lens to do it. The point of concourse behind the lens must 

 fall critically upon the retina, or the vision is confused ; yet 

 other things remaining the same, this point, by the immuta 

 bio ] roperties of light, is carried further back when the ray? 

 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 his screw ; but 

 how is this to be managed 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 refraction of light be maintained, some alteration in the 

 state of the organ there must be — had long formed a subject 

 of inquiry and coiijecture. The change, though sufficient 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 structure and operation of the 

 i>rgan, seem at length to have ascertained the mechanical 

 ulteration which the parts of the eye undergo. It is found, 

 that by the action of ccrtaui muscles called the straight mus- 

 cles,^' and which action is the most advantageous that could 

 hi) imagined for the purpose — it is found, I say, that when- 

 ever the eye is directed to a near object, three changes are 

 produced in it at the same time, all severally contributing to 

 the adj ustment required. The cornea or outermost coat of the 

 eye is rendered more round and prominent, the crystalline 

 lens underneath is pushed forward, and the axis of vision, 

 * Plate L, Fig. 2. There are four straight muscles, a, a, belong to 

 ihe globe of the eye, each arising from the bottom of the orbit, where 

 they surround c, the optic nerve. They are strong and fleshy, and 

 are inserted by broad thin tendons at the fore part of the globe of the 

 eye into the tunica sclerotica. Their use is to turn the eye in differ- 

 ent directions ; hence they .are severally named levator oculi, depres- 

 sor oculi, adductor oculi, and abductor oculi. 



Nat. Theol. 2 



