106 MAGNIFICATION AND MICROMETRY [CH. IV 



The spaces between the lines should be y 1 -^ and ^\ Ti mm. (or if in 

 inches, T -^ ¥ and twit m -) Micrometers are sometimes ruled on the 

 slide, but more satisfactorily on a cover-glass of known thickness, 

 preferably o. 15-0. 18 mm. The covers should be perfectly clean before 

 the ruling, and afterwards simply dusted off with a camel's hair 

 duster, and then mounted, lines downward over a shellac or other 

 good cell. (See Ch. VII). 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 un- 

 covered one cannot use the micrometer with an oil immersion, as the 

 oil obliterates the lines. Cleaning the slide makes the lines less sharp 

 as stated. If the lines are very coarse, it is an advantage to fill them 

 with plumbago. This may be done either with some very fine plum- 

 bago on the end of a soft cork, or by using an exceedingly soft lead 

 pencil. Lines properly filled may be covered with balsam and a cover- 

 glass as in ordinary balsam mounting (Ch. VII). 



§ 160. Determination of Magnification. — This is most readily 

 accomplished by the use of some form of camera lucida (Ch. V), 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 very readily determined (Fig. 97, S 162)- 



Employ the 16 mm. ( -'3 in. ) objective and a 37 mm. (or x S ocu- 

 lar with a stage micrometer as object. For this power the yV mm. 

 spaces of the micrometer should be used as object. Focus sharply. 



Fig. 96. Diagram of a stage micrometer, 

 with a ring on the lines to facilitate finding them. 



It is somewhat difficult to find the mi- 

 crometer lines. To avoid this it is well to 

 have a small ring enclosing some of the 

 micrometer lines (Fig. 96). The light must also be carefully regu- 

 lated. If too much light is used, i. e., too large an aperture, the lines 

 will be drowned in the light. In focusing with the high powers be 

 very careful. Remember the micrometers are expensive, and one can- 

 not afford to break them. As suggested in § 74, focus on the edge of 

 the cement ring enclosing the lines, then in focusing down to find 

 the lines, move the preparation very slightly, back and forth. 



After the lines are sharply focused, and the slide clamped in posi- 

 tion make the tube of the microscope horizontal, by bending the flexible 

 pillar, being careful not to bring any strain upon the fine adjustment 

 (frontispiece ). 



