MEASUREMENT OF OBJECTS 23 



divisions of this scale varies with 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 de- 

 termine the value of divisions of the scale under a given 

 combination of glasses, a stage micrometer, having lines 

 drawn to T ^Voths of an inch apart, should be placed on 

 the stage, and focussed under the objective and eye- 

 piece whose magnifying power 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 micro- 

 meter covers the intervals between six lines of the eye- 

 piece micrometer, the former being T^Vo"th f an mc h 

 apart, the interval between two lines of the latter (with 

 that combination of lenses) will correspond to -^tro^h f 

 an inch, and the linear measurement of any object 

 which fills such an interval under that combination of 

 lenses will be suV^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. 



