The microscope; an introduction to microscopic methods and to histology . CH. IV] MAGNIFICATION AND MICROMETRY 107 Fig. 97. IVollaslons Camera Lu-cida, showing the rays from the micro-scope and from the drawing surface,also the position of the pupil of the eye. Axis, Axis. Axial rays fromthe microscope and from the drawingsurface (Ch. V). Camera Lucida. A section of thequadrangular prism showing thecourse of the rays in the prism fromthe microscope to the eye. As the raysare twice reflected, they have the samerelation on entering the eye that theywould have by looking directly into theocular.
The microscope; an introduction to microscopic methods and to histology . CH. IV] MAGNIFICATION AND MICROMETRY 107 Fig. 97. IVollaslons Camera Lu-cida, showing the rays from the micro-scope and from the drawing surface,also the position of the pupil of the eye. Axis, Axis. Axial rays fromthe microscope and from the drawingsurface (Ch. V). Camera Lucida. A section of thequadrangular prism showing thecourse of the rays in the prism fromthe microscope to the eye. As the raysare twice reflected, they have the samerelation on entering the eye that theywould have by looking directly into theocular. A. B. The lateral rays from themicroscope and their projection uponthe drawing surface. C. D the drawingsurface to ths eye. A. D. A D. Overlapping portions of the two fields, where both the micro-scopic image and the drawing surface, pencil, etc., can be seen. It is representedby the shaded part of the overlapping circles at the right. Ocular. The ocular of the microscope. P. The drawing pencil. Its point is shown in the overlapping; Fig. 97. Put a Wollaston camera lucida (Fig. 97 and Ch. V) in position,and turn the ocular around if necessary so that the broad flat surfacemay face directly upward, as shown in Fig. 97. Elevate the micro-scope by putting a block under the base, so that the perpendicular dis-tance from the upper surface of the camera lucida to the table is 250mm. (§ 162). Place some white paper on the work-table beneath thecamera lucida. Close one eye, and hold the head so that the other may be veryclose to the camera lucida. Look directly down. The image will ap-pear to be on the table. It may be necessary to readjust the focusafter the camera lucida is in position. If there is difficulty in seeingdividers and image consult Ch. V. Measure the image with dividersand obtain the power exactly as above (§156-157). Thus : Suppose two of the TVth mm., spaces were taken as object,and the image is measured by the dividers, and the spread of thedivide
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