. The microscope and microscopical methods. Microscopes; 1896. CH. ] 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. (73 in.) objective and a 50 mm. (2 in., A. or No. 1) ocular and stage micrometer as object. For this power the TVth 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. Wo
. The microscope and microscopical methods. Microscopes; 1896. CH. ] 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. (73 in.) objective and a 50 mm. (2 in., A. or No. 1) ocular and stage micrometer as object. For this power the TVth 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 drawing surface, and the position of the pupil of the eye. Axis, Axis. Axial rays from the •microscope and from the drawing sur- face (Ch. V). Camera Lucida. A section of the quadrangular prism showing the course of the rays in the ptism from the microscope to the eye. As the rays are twice 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 drawing surface. CD. Rays from the drawing sur- face to the eye. A D, A' D'. Overlapping portion of the two fields, where both the microscopic linage 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 pa
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