170 



drawing paper ; mark the spaces of the micrometer 

 as projected upon the paper and determine how 

 many of the divisions of the rule are contained 

 within one or more spaces on the paper. If the 

 values are in millimeters and it should be found 

 that 25.0 mm. on the rule are contained in one 

 space on the paper, the magnification would be 

 250. If 18.6 on the rule are contained within 

 three spaces on the paper, the magnification would 

 be 63 times. 



To Measure the Size of an Object. One of 



the most valuable possibilities of the microscope is 

 to be able to accurately measure the actual size of a 

 minute object. Computations may be made in 

 inches or millimeters by figuring 25.4 mm. equal 

 to 1 inch. It may be done by several methods, 

 two of which are generally followed. 



The first of these give satisfactory results on 

 coarser objects and wherever the most accurate 

 results are not required, although it is somewhat 

 inconvenient. The requisites are 



A camera lucida. 



A stage micrometer. 



The object is placed upon the stage and after 

 focusing, the camera lucida is attached and the 

 instrument set up exactly as for drawing. On the 

 drawing paper the outlines of object, or that por- 

 tion which is to be measured are marked, without 



