424 Mr. Coddington on the Improvement 



Now in a telescope, z being nearly equal to the semi-aperture 

 of the field glass, is very much less than /, to which k is equal, 

 and as a high magnifying power is produced by means of a 

 powerful eye-piece, applied to an object-glass which is never 

 changed, and that the apertures of the lenses used for eye- 

 pieces, of the same kind, are usually proportional to their focal 

 lengths, the higher the magnifying power, the less is the fraction 



For instance, in a 5 foot telescope, it is seldom, if ever, greater 

 the quantity 



than — T— r , and often very much less, so that the value of 



14400 



\z- • . . 1 

 -T-^. IS about 



2/' """""^ 7200* 



In a microscope, on^^the other hand, / is a very small quantity, 

 though k is not so, and the magnifying power is raised by 

 applying an object-glass of shorter focus to the same body. 

 The following values are, I believe, such as might fairly occur: 



20 

 1 



k = 3. 



\z- 1 



These give ^ = 55^ which as, with different object-glasses, 



z and k are constant, and x usually proportional to / may be 

 considered as its general value. 



