<288 optics. 



If the lens C were but of half an inch focal 

 distance, the lengths of the objects would be mag- 

 nified twice as much ; the surfaces four times, and 

 the magnitude or bulk, eight times as much as 

 before. 



If the focal distance of the lens C be a quarter 

 of an inch, then the lengths are magnified four 

 times as much, viz. twenty-four, twenty-eight, or 

 thirty-two times; the surfaces sixteen times as 

 much, or 576 times more than to the naked eye at 

 six inches distance; and solid bodies are magnified 

 sixty-four times more than by the lens of a whole 

 inch focal distance. 



Again ; suppose the lens so small that its focal 

 distance is but one-tenth of an inch, then the 

 length of an object is magnified sixty, seventy, or 

 eighty times; the surface 3600, 3900, or 6400 

 times; and the solidity, or whole bulk, 216,000, 

 343,000, or 512,000 times, or so much larger do 

 the bodies of mites, or their eggs, appear than to 

 the naked eye at six, seven, or eight inches 

 distance. 



After the same manner you may compute the 

 magnifying power of lenses of one-twentieth, one- 

 thirtieth, one-fortieth, and even one-fiftieth part of 

 an inch focal distance, which mav be made, if re- 

 quired; but they are with difficulty used. By a 

 lens one-fiftieth of an inch, the length is magni- 

 fied 300 times, and the surface 90,000 times, and 

 the solidity 27,000,000 times. 



But these enormous powers of magnifying are 

 much better effected in compound microscopes; 

 and be that in what degree you please, it is thus 

 easily estimated. Let C be the object-lens in the 

 cell Q (Fig. 2.) of the compound microscope; if, 

 then, a small object, a b, be placed on the stage at 

 a little more than the focal distance, there will be 



