Ch. V] 



MEASURING WITH THE MICROSCOl'K 



145 



eter by the number of divisions on the ocular micrometer required to 

 include them, and the quotient so obtained will give the valuation of 

 the ocular micrometer. For example, suppose the millimeter is taken 

 as the unit for the stage micrometer and this unit is divided into spaces 

 of 0.1 and 0.01 millimeters. If with a given optical combination and 

 tube-length it requires 10 spaces on the ocular micrometer to include 

 the real image of 0.1 millimeter on * ~ 



the stage micrometer, obviously one 

 space on the ocular micrometer in- 

 cludes only one-tenth as much, or 0.1 

 mm. -5- 10 = 0.01 mm. That is, each 

 space on the ocular micrometer in- 

 cludes 0.01 of a millimeter on the 

 stage micrometer, or 0.01 millimeter 

 of the length of any object under the 

 microscope, the conditions remaining 

 the same. Or, in other words, it re- 

 quires 100 spaces on the ocular mi- 

 crometer to include 1 millimeter on 

 the stage micrometer; then, as be- 

 fore, 1 space of the ocular micrometer 

 would have a valuation of 0.01 milli- 

 meter for the purposes of micrometry. 

 The size of any minute object may be 

 determined by multiplying this valua- 

 tion of one space by the number of 

 spaces required to include it. For example, suppose the fly's wing 

 or some part of it covered 8 spaces on the ocular micrometer; it 

 would be known that the real size of the part measured 0.01 mm. 

 X 8 = 0.08 mm. or 80 [x, (§ 246). 



Proceed in exactly the same manner to get the ocular micrometer 

 valuation when using any objective whether it is of higher or lower 

 power than the one in this section. 



Any Huygenian ocular may be used as a micrometer ocular by 

 placing the ocular micrometer at the level of the ocular diaphragm 

 where the real image is formed. If there is a slit in the side of the 



Fig. 90. The Images of the 

 Ocular and of the Stage 

 Micrometer, Showing how to 

 Arrange the Lines. 



o.m Ocular, s.m Stage mi- 

 crometer lines. 



A Lines of the ocular mi- 

 crometer opposite the middle of 

 the lines of the stage micrometer. 



B Lines of the ocular mi- 

 crometer at the right side of the 

 lines of the stage micrometer 

 (compare fig. 98). 



