Measuring Devices Used on n Microsrope 



10 



20 



30 



05 

 40 



10 



Figure 22. A xiernier scale. 



number on the long scale, then the number is recorded as a whole 

 nianber with no decimals follo^ving it. The decimal is determined by 

 the point of coincidence of the line of the short scale which perfectly 

 coincides with any line on the long scale (.5 of the short scale in Fig. 

 22). The reading, therefore, in Fig. 22 will be 21.5. 



Measuring Objects with an Ocular Micrometer 



The measurement of slide specimens usually is done with a micrometer 

 disc placed in the ocular, but first the disc's actual value with respect to 

 the magnification at which it is being used has to be calibrated against 

 a stage micrometer. Stage micrometers usually have a 2 mm. scale 

 divided into .01 mm. divisions, or a 0.2 inch scale divided into .001 inch 

 divisions. The ocular micrometer has a 5 mm. scale divided into 50 (0.1 

 mm.) divisions or 100 (.05 mm.) divisions. 



Unscrew the top lens of the ocular, place the ocular micrometer on 

 the circular shelf inside and replace the lens. Focus on the stage mi- 

 crometer, moving it until the zero line of both micrometers coincide. 

 Then a definite distance on the stage micrometer will be made to 

 correspond to a certain ntimber of divisions on the ocular micrometer. 



Tiie rest is simple; for example: 



The ocular micrometer has 100 divisions. 



Suppose that 30 divisions (each measuring .01 mm.) of the stage micrometer 

 equal the 100 ocular micrometer divisions. Then 30 x .01 mm. = .30 mm., 

 the length of the 100 divisions on the ocular micrometer. 



Next, .30 -i- 100 = .003 mm. (3.0 /^), the value of each single division. 



Therefore, when the ocular micrometer (each division measuring 3.0 jj) is 

 focused on a specimen, and if the specimen requires 9 divisions to meet its 

 length, multiply 9 by 3.0 /^ to equal 27.0 /.t (.027 mm.), the total length of the 

 specimen. 



