41 



pencil and outline upon the paper one of the 

 spaces. By dividing the actual number of divi- 

 sions on the rule into this enlarged space the exact 

 magnifying power will be determined. Thus, if it 

 is found that five spaces are contained in the one 

 space on the paper, the magnifying power is five and 

 the focus of the lens 2 inches, or if ten spaces, it is 

 ten with a focus of 1 inch. One or several lenses 

 in conjunction may be examined in this way. 

 Some difficulty may or probably will be experi- 

 enced in seeing the divisions on the rule 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 difficulty may be experienced, 

 it will be found that after a little earnest effort, 

 both eyes unconsciously remain open and the 

 prominence with which objects have appeared to 

 the unoccupied eye is less, as the mind becomes 

 intent upon the object it is viewing.. 



