136 



MAGNIFICATION OF THE MICROSCOPE 



[Ch. V 



The spaces between the lines should be 0.1 and 0.01 mm. (or if in 

 inches, 0.01 and 0.001 in.). Micrometers are sometimes ruled on the 



slide, but more satisfactorily 

 on a cover-glass of known 

 thickness, preferably 0.15- 

 0.18 mm. The covers 

 should be perfectly clean 

 before ruling, and after- 

 wards simply dusted off 

 with a camel's hair duster, 

 and then mounted, lines 

 downward over a shellac or 

 other good cell (see Ch. X). 

 If one rubs the lines the 

 edges of the furrow made 

 by the diamond are liable to 

 be rounded and the sharp- 

 ness of the micrometer is 

 lost. If the lines are on the 

 slide and uncovered one cannot use the micrometer with an oil im- 

 mersion, as the oil obliterates the lines. Cleaning the slide makes the 

 lines less sharp, as stated. If the lines are coarse, it is an advantage 

 to fill them with plumbago or graphite. This may be done with 

 some very fine plumbago on the end of a soft cork, or by using a soft 

 lead pencil. Lines properly filled may be covered with balsam and 

 a cover-glass as in ordinary balsam 

 mounting (Ch. X). 



§ 234. Determination of magnifica- 

 tion. — This is most readily accomplished 

 by the use of some form of camera lucida, 

 that of Wollaston being most convenient, 

 as it may be used for all powers, and the 

 determination of the standard distance 



of 250 millimeters at which to measure the images is readily accom- 

 plished (fig. 81). 



Employ the 16 mm. objective and a 4X or 5X ocular with a stage 



Fig. 81. Wollaston's Camera Lucida. 



Fig. S2. 

 eter with 

 Lines. 



Stage Microm- 

 a Ring on the 



