MAGNIFYING POWERS. 



135 



By the use of the preceding- table one is able to meas- 

 ure the focal length of any objective, and without in- 

 volving the trouble of computations. In ordinary cases 

 it will be only necessary to see that the two microme- 

 ters shall be exactly ten or twelve and one-half inches 

 apart. The amplification in either case being noted, we 

 enter the table under the proper head, and correspond- 

 ing to the amplification will be found the focal distance. 

 For instance, taking the first case we have presented, 



