16 PRACTICAL BOTANY 



the combination of lenses used ; accordingly, before the 

 micrometer can be employed in the actual measurement of 

 objects, the value of the divisions must be determined for each 

 combination, and a table of the results should be kept for 

 reference in the case of the microscope. To determine the 

 value of divisions of the scale under a given combination of 

 glasses, a stage micrometer, having lines drawn to TD ^oths of 

 an inch apart, should be placed on the stage, and focused 

 under the objective and eyepiece whose magnifying powers it is 

 desired to measure : the relation of the divisions of the stage 

 micrometer (these intervals being of known value) to those of 

 the eye-piece micrometer is then to be noted. Suppose that 

 the interval between two lines of the stage micrometer covers 

 the intervals between six lines of the eye-piece micrometer, the 

 former being y^njth of an inch apart, the interval between two 

 lines of the latter (with that combination of lenses) will corre- 

 spond to smooth of an inch, and the linear measurement of any 

 object which fills such an interval under that combination of 

 lenses will be g-^jg-th of an inch. It is usual to state the size of 

 objects seen under the microscope according to the linear 

 measurement of the diameter. The simpler method of 

 measurement by laying the stage micrometer inverted on the 

 slide carrying the object to be measured, though direct, is 

 open to many objections, and can at best only be used with low 

 powers. 



