CH. Vj MAGNIFICATION OF THE MICROSCOPE 137 



micrometer as object. For this power the o.i mm. spaces of the 

 micrometer should be used as object. Focus sharply. 



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. 82). 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. Re- 

 member the micrometers are expensive and one cannot afford to break 

 them. As suggested above, focus on the edge of the cement ring en- 

 closing the lines; then, in focusing down to find the lines, move the 

 preparation very slightly, back and forth. This will bring the lines 

 into the field and the shadow made by them will indicate their pres- 

 ence, and one can then focus until they are sharp. 



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 (fig. 25). 



Put a Wollaston camera lucida (fig. 81) in position, 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 perpendicular distance 

 from the upper surface of the camera lucida to the table is 250 mm. 

 ( 236). Place same 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 

 appear to be on the table. It may be necessary to readjust the focus 

 after the camera lucida is in position. If there is difficulty in seeing 

 both dividers and image, consult Ch. VI. Measure the image with 

 dividers and obtain the power exactly as above ( 231). 



Thus: suppose two of the o.i mm. spaces were taken as object 

 and the image is measured by the dividers, and the spread of the 

 dividers is found on the steel rule to be 9.4 millimeters, the mag- 

 nification (which is the ratio between size "of image and object) is 

 9.4 -r- 0.2 = 47. That is, the magnification is 47 diameters, or 47 

 times linear. 



