CH. IV] 



MAGNIFICATION AND MICROMETRY 



107 



Fig. 97. IVollaslon's 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 from 

 the microscope and from the drawing 

 surface (Ch. V). 



Camera Lucida. A section of the 

 quadrangular prism showing the 

 course of the rays in the prism 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 

 microscope and their projection upon 

 the drawing surface. 



C. D Rays '.from the drawing 

 surface 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 represented 

 by 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; fields. 



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 surface 

 may 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 250 

 mm. (§ 162). 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 ap- 

 pear 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 image consult Ch. V. Measure the image with dividers 

 and obtain the power exactly as above (§156-157). 



Thus : Suppose two of the T Vth 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 9f millimeters. If now the 

 object is y-fjths of a millimeter and the magnified image is 9A milli- 

 meters, the magnification (which is the ratio between size of object 



