Ch. V] 



MEASURING WITH THE MICROSCOPE 



147 



can then count up the number of spaces on the ocular micrometer 

 required to measure one or more spaces of the stage micrometer. 

 To this is then added the y^tt spaces on the drum. For example 

 suppose that three o.oi mm. spaces of the stage micrometer are taken 

 as object, and that it requires 

 seven complete spaces of the 

 ocular micrometer and yVu on 

 the drum to include the three 

 spaces on the stage micrometer; 

 then each space on the ocular 

 micrometer would be equal to 

 0.03 mm. divided by 7.50=0.004 

 mm. or 4 ju.. One of the spaces 

 on the drum which represents 

 one-hundredth of an interval on 

 the ocular micrometer would 

 have a valuation under these 

 conditions of 4 /u. divided by 

 100 = 0.04 microns. This gives 

 a notion of the minuteness of 

 the object which can be meas- 

 ured, and of the smallness of 

 the error in measuring large ob- 

 jects, even if the observation 

 erred in getting the object one 

 or more of the drum divisions 

 too large or too smaU. 



For an actual measurement 

 with this ocular micrometer, see 

 (§ 251). 



Fig. 92. Field of the Microscope 

 Showing the Movable Scale of 

 the huygenian micrometer ocu- 

 LAR (Fig. 91). 



The arrow indicates that the scale may 

 be moved in both directions. 



0,5,10,15,20 These figures indicate 

 the 20 spaces in groups of 5. Each 

 space represents a total revolution o£ 

 the screw (screw with j mm. pitch). 

 Each o£ the 100 divisions on the drum 

 (fig. 91) represents then jj^ mm. 



Object A circular object covering five 

 of the micrometer spaces, and the drum 

 shows 45 division to measure the partial 

 space; the entire object then measures 

 5.45 divisions. 



One would proceed exactly as above for getting the valuation with 

 any other objective. 



FiLAa Ocular Micrometer 

 § 243. This form of ocular micrometer usually consists of a Rams- 

 den ocular with fixed cross Hues and a movable line (fig. 94). 



