ADJUSTMENT OF THE EYE FOR VARYING DISTANCES. 427 



has served as a guide to Art ; or, in other words, the Divine 

 Artificer has thus condescended to teach the human workman. 



550. There is another wonderful arrangement in the 

 healthy Eye, which the optician can only imitate in his 

 instruments in a very bungling manner. It is that by which 

 the eye adapts itself to view objects at different distances 

 from it, with equal distinctness. If we look at a near object 

 with a Telescope, adjusting the instrument so as to see it dis- 

 tinctly, and then turn it towards a remote object, we shall 

 not see the latter with equal clearness until the instrument 

 has been again adjusted. If we then turn it back to the 

 nearer object, we shall find that the change in the adjustment 

 occasions the representation of it to be now indistinct ; and 

 in order to bring back the image to its former clearness, it is 

 requisite to re-adjust the instrument to its first condition. 

 This is a necessary consequence of the optical law, that the 

 distance of the image from the lens which forms it, varies 

 with that of the object, being increased as the object is 

 brought nearer, and diminished as it recedes. If the Eye 

 were constructed in the same manner, we should not be able 

 to see distinctly, without the aid of artificial assistance, at any 

 other distance than that for which it is adjusted. Hence if a 

 perfect picture of an object situated at twelve inches' distance 

 from the eye, were formed upon the retina, we should not be 

 able to see it clearly when brought to the distance of six inches, 

 nor when removed to the distance of six feet ; because in the 

 first of these cases the rays would not be brought to a focus 

 upon the retina, but at a point behind it (if they were 

 allowed to pass on unchecked) ; whilst in the second, they 

 would be brought to a focus at a point nearer than the retina, 

 and would consequently begin to separate again before they 

 reach it. 



551. But the healthy eye possesses a power of perfect 

 adjustment to the viewing of objects situated at different 

 distances ; and this without any effort or intention on our 

 parts, but, as it were, by an instinctive operation. That such 

 a change really takes place, we may readily convince ourselves, 

 by looking at a near and at a distant object placed in the 

 same line, a pencil-case, for instance, held up at a few inches 

 from the eye, and a chimney half a mile off. We shall find 

 that no effort of attention will enable us to see them both 



