3 
Grroat alterations in tlie focal lengths of both objectives and eye-pieces would 
have been necessary in order to designate these by the same convenient round 
numbers as in the new series. Our microscopes, however, being in such extensive 
use, so many microscopists have become accustomed to the focal lengths hitherto 
adopted and the usual denotation of the various combinations, that a radical alteration 
in this direction might give rise to considerable confusion. For these reasons the 
former designation of the objectives by letters and the arbitrary numeration of the 
eye-pieces has been retained in the older series. 
