will be exactly in focus. Place the ruler so that it will 

 not cover much of the lens. It is immaterial what the 

 divisions on the ruler are, whether inches or milli- 

 meters, so long as they are reasonably fine. View the 

 divisions with the right eye and open the left eye, when 

 it will be found that the divisions are apparently pro- 

 jected upon the paper. Take a pencil and outline one 

 of the spaces upon the paper. By dividing this 

 enlarged space by the actual number of divisions on 

 the ruler, the exact magnifying power will be determ- 

 ined. Thus, if it is found that the one space on the 

 paper contains five spaces on the ruler, the magnifying 

 power is five, and the focus of the lens 2 inches ; or if 

 ten spaces, it is ten, with a focus of i inch. One or 

 several lenses in conjunction may be examined in this 

 way. Some difficulty may and probably will be 

 experienced in seeing the divisions on the ruler and 

 on the paper at the same time, but this will be over- 

 come with a little practice. Indeed, it is well to point 

 out at this stage that both eyes should be kept open in 

 viewing objects through simple as well as compound 

 microscopes, as continued work can be done with 

 infinitely more comfort and, while at first some diffi- 

 culty may be experienced, it will be found that after 

 a little earnest effort, both eyes unconsciously remain 

 open and the prominence with which objects appear 

 to the unoccupied eye diminishes as the mind becomes 

 intent upon the object it is viewing. 



26 



