optics. 289 



formed by the said lens C, a large image S T, in 

 the upper part of the microscope; and this image 

 is viewed through the glass G H in its focus below 

 atO. 



Now it is easy to understand, that the image 

 S T is as much larger than the object a b, as the 

 distance C A exceeds the distance C a from the 

 lens. Suppose the image S T six times larger than 

 the object a b; then if it be viewed by a lens G H 

 of one inch focal distance, the image S T will ap- 

 pear magnified six times at least, and therefore the 

 object a b will be magnified six times six, or 36 

 times in length ; and 36 times 36, or 1296 times in 

 surface; and 36 times 1296, or 46,656 times in the 

 bulk of solidity. And yet with these great powers 

 of magnifying, the lens C may not be of less than 

 half an inch focal distance, in the least sort of 

 compound pocket microscopes. 



But with a single eye-glass G H, we have too 

 small a field of view ; therefore we use two, viz. 

 A B and D E; the first contracts the image S T 

 into another, I M, which is less; and this is viewed 

 by the eye-glass D E. Now both these glasses 

 may have a magnifying power equal to that of a 

 given single glass G H, by this rule: Let their 

 distance be equal to the difference of their focal 

 lengths, and their magnifying power will be equal 

 to that of a lens, whose focal distance is half that 

 of the greater A B. 



For example — Suppose the focal length of A B to 

 be 2£ inches, and that of D £ 1 inch; then if 

 their distance be l£ inch, their joint magnifying 

 power will be equal to that of a single lens G H, 

 whose focal distance is 1 1 equal half that of A B. 

 By the two eye-glasses the rays are converged to 

 the eye at the compound focus F, much less 



vol. i. u 



