120 MAGNIFICATION AND MICROMETRY [CH. IV 



used for all powers, and the determination of the standard distance 

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

 mined (Fig. 108, § 178). 



Employ the 16 mm. (fi in.) objective and a 37 mm. (orX8 

 ocular with a stage micrometer as object. For this power the T V 

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

 sharply. 



Fig. 107. Abbe's Test Plate to show the enclosure of the micrometer lines 

 by small rings. See also Fig. 75. 



It is somewhat difficult to find the micrometer lines. To avoid 

 this it is well to have a small ring enclosing some of the micrometer 

 lines (Fig. 107). The light' must also be carefully regulated. 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 § 83, 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 

 position make the tube of the microscope horizontal, by bending the 

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

 adjustment (frontispiece).. 



Put a Wollaston camera lucida (Fig. 108 and Ch. V) in posi- 

 tion, and turn the ocular around if necessary so that the broad flat 

 surface may face directly upward, as shown in the figure. Elevate 

 the microscope by putting a block under the base, so that the per- 

 pendicular distance from the upper surface of the camera lucida to 

 the table is 250 mm. (§ 178). Place some white paper on the 

 work-table beneath the camera lucida. 



Close one eye, and hold the head so that the other may be very 

 close to the camera lucida. Look directly down. The image will 



