OPTICS. 573 



will by the lens be united in a point K, between 

 the lens D E and its focus F : if then the eye be 

 placed at K, it will take into its view an object 

 greater than the lens D E. 



Again, let G H be a portion of an object A B, 

 less than the lens D E j draw G D, HE, which 

 will be diverging rays, and, therefore, will be 

 united at a point I, farther distant from the lens 

 than the focus F ; hence, if an eye be placed far- 

 ther from the lens than its focal distance, it can 

 never see any object, or part of an object, at one 

 view, so large as the lens, but always smaller. 

 And, universally, the visible part of an object will 

 be to the lens, as the focal distance of the lens to 

 the distance of the eye. 



Since, then, it is evident, the nature of a con- 

 vex lens is such as will render an object distinctly 

 visible to the eye at the distance of its focus, the 

 reason why they are used as microscopes is very 

 plain. For, suppose the distance, A B (Fig. 8. 

 and 9.) be six inches, where the naked eye, B, can 

 but just perceive the object A distinctly, and let 

 the focal distance C D of the lens D be half an 

 inch ; then since C D is but one-twelfth of A B, 

 the length of the object at C will appear twelve 

 times as large as at A : if it were a surface, it 

 would be 144 times as great ; and the solidity, or 

 bulk, would be magnified 1728 times. 



If C D, the focal distance of the lens D, be but 

 one-fourth part of an inch, then will that be but 

 one twenty-fourth of A B, equal six inches, and 

 the length of the objects will be magnified 24 

 times; the surface 576 times, and the solidity 

 13,824 times ; for those numbers are the square 

 and cube of 24. Whence it appears, that single 

 glass lenses make very good microscopes, and have 



vol. 1. t 



