CH. IV] 



MAGNIFICATION AND MICROMETRY 



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 image consult Ch. V. Measure the image with dividers 

 and obtain the power exactly as above (§ 172-173). 



Fig. 108. Wollasion's Camera 

 Lucida, showing the rays from 

 the microscope and from the draw- 

 ing 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 the eye. 



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

 ■microscopic image and the drawing surface, pencil, etc., can be seen. It is rep- 

 resented 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. 



10S 



Thus: Suppose two of the y 1 ^ 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 gf millimeters. If the ob- 

 ject is T 2 7 of a millimeter and the magnified image is of millimeters, 

 the magnification (which is the ratio between size of object and 

 image) is of -7-^—47. That is, the magnificatfon is 47 diameters, 

 or 47 times linear. If the fractional numbers in the above example 

 trouble the student, both may be reduced to the same denomination, 

 thus: If the size of the image is found to be of mm. this number 

 may be reduced to tenths mm., so it will be of the same denomina- 

 tion as the object. In 9 mm. there are 90 tenths, and in f there 

 are 4 tenths, then the whole length of the image is 90-1-4=94 tenths 



