CH. IV] 



MAGNIFICATION AND MICROMETRY. 



95 



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 image is very readily determined (Fig. 92, § 153). 

 Employ the 16 mm. { 2 /i in.) objective and a 50 mm. (2 in., A. or 

 No. 1) ocular and stage micrometer as object. For this power the y^th 

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

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



Fig. 92. Wollaston' s Camera Luci- 

 da, showing the rays from the micro- 

 scope and from the draiving surface, 

 and the position of the pupil of the eye. 



Axis, Axis. Axial rays Jrom the 

 microscope and from the draiving sur- 

 face { Ch. V). 



Camera Lucida. A section of the 

 quadrangular prism showing the 

 course of the rays in the pt ism from 

 the microscope to the eye. As the rays 

 are tzvice reflected, they have the same 

 relation on entering the eye that they 

 would have by looking directly into the 

 ocular. 



A B. The lateral rays from the mi- 

 croscope and their projection on the 

 draiving surface. 



CD. Rays from the drawing sur- 

 face to the eye. 



Fig. 92. 



A D, A' D' . Overlapping portion of the two fields, where both the microscopic 

 image and the drawing surface, pencil, etc., may be seen. It is represented by the 

 shaded part in the overlapping circles at the right. 



Ocular. The ocular of the microscope. 



P. The drawing pencil. Its point is shown in the overlapping fields. 



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

 (Frontispiece). 



Put a Wollaston camera lucida (Ch. V.) in position, and turn the oc- 

 ular around if necessary so that the broad flat surface may face directly 

 upward, as shown in Fig. 92. 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. (§ 153). 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. L,ook 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 dividers and 



