300 optics. 



be divided by the latter, the quotient will express 

 the magnifying power. 



This telescope may be made to magnify in any 

 given degree, provided it be of a sufficient length. 

 For the greater the focal distance of the object- 

 glass, the less may be the focal distance of the 

 eye-glass, though not directly in proportion. Thus, 

 an object-glass of ten feet focal distance will ad- 

 mit of an eye-glass whose focal distance is little 

 more than two inches and a half; which will mag- 

 nify near forty-eight times; but an object-glass of 

 one hundred feet focus will require an eye-glass 

 somewhat more than six inches ; and will, therefore, 

 magnify almost two hundred times. 



A telescope for viewing terrestrial objects should 

 be constructed so as to show them in their natural 

 posture. And this is done by one object-glass 

 c d (Fig. 3.), and three eye-glasses e f, g It, i k, so 

 placed, that the distance between any two, which 

 are nearest to each other, may be equal to the sum 

 of their focal distances, as in the figure, where the 

 focus of the glasses c d and e /"meet at F, those of 

 the glasses e f and g h, meet at /, and of g h and 

 i k at m; the eye being at n, in or near the focus 

 of the eye-glass i k, on the other side. Then it is 

 plain, that these pencils of rays which flow from the 

 object A B, and pass through the object-glass c d, 

 will meet and form an inverted image C F D in the 

 focus of that glass; and the image being also in the 

 locus of the glass e f, the rays of the pencils will 

 become parallel, after passing through that glass, 

 and cross at /, in the focus of the glass e f; from 

 whence they pass on to the next glass g h, and by 

 going through it, they are converged to the points 

 in its other focus, where they form an erect image 

 E m F, of the object A JB ; and as this image is also 



