optics. 287 



nearer to, or farther from, the eye ; because the 

 nearer it is, the larger is the visual angle under 

 which it appears, and vice versa. 



But the eye is so formed as to admit of distinct 

 vision by such rays only as are nearly parallel j and 

 therefore every object must be removed to such a 

 distance from the eye, that the rays of light issuing 

 from every point thereof, may fall upon the eye 

 with small divergency, or nearly parallel; which 

 distance is, in different eyes, from six to eight, 

 inches, as any one may find by trial. 



Now, since a convex glass converges parallel 

 rays to a point or focus ; therefore, on the contrary, 

 if an object be placed in the focus of such a lens, 

 the rays proceeding from each point will be re- 

 fracted parallel to the eye, and thereby produce 

 distinct vision of that object in its focus. 



Hence, then, it follows, that if a b be a very 

 small object in the focus of the glass C (Fig. 3.) 

 whose focal distance is one inch, the eye applied to 

 the lens C will have the distinct vision of it ; and 

 this being at a distance six, seven, or eight times 

 nearer than the eye alone could clearly see it, it 

 must appear so many times larger than to the naked 

 eye ; and therefore, we properly say, it is magni- 

 fied to such eyes, six, seven, or eight times in 

 length and breadth. 



But all surfaces are magnified in proportion to 

 the squares of their lengths or sides ; therefore the 

 surfaces of objects are magnified thirty- six, forty - 

 nine, or sixty-four times, by a lens of one inch 

 focal distance. 



Also the bulk or magnitude of the whole body, 

 will be magnified in proportion to the cubes of the 

 sides or length ; and therefore all solid bodies will, 

 by such a lens, be magnified 316,343, or 512 times 

 to such eyes respectively. 



