MEASUREMENT OF OBJECTS 121 



this enables the observer by turning a milled screw to place with ease and 

 accuracy any portion of the slide under the objective, and by continuous 

 rotation, rapidly to examine the whole preparation.] 



SECTION V. THE MEASUREMENT OF MICROSCOPICAL OBJECTS. 



1. The experimental determination of the magnification 

 produced by a system of lenses. 



Magnification produced by an optical system is of course magnification 

 in diameters. 



Microscope makers supply a table with each of their instruments, showing 

 with a given tube length the magnification produced with every combination 

 of objective and ocular. This table may be verified roughly by one or other 

 of the two following methods : 



A. With a camera lucida. For this purpose a camera lucida and a stage 

 micrometer are necessary. A stage micrometer is a thin glass slide, on 

 which a scale mechanically divided by parallel lines into T J^ths of a 

 millimetre has been engraved. 



1. Select the eyepiece and the objective of which the magnification produced 

 by the combination is to be determined. Lengthen the tube to 160 mm., 

 or whatever is the proper working length. Place the micrometer slide on 

 the stage and get it into focus, so that the divisions on the scale are sharply 

 defined. 



2. Place a sheet of paper, bluish for choice, on a small drawing table level 

 with the stage of the microscope on the right-hand side of the instrument. 

 Fit the camera lucida to the eyepiece. 



3. On looking down the tube of the microscope, two images of the scale 

 on the micrometer will be seen one formed directly by rays passing through 

 the camera lucida, the other projected by reflection at the prism on to the 

 paper. If the image projected on to the paper be approached with the point 

 of a pencil, the latter will also come into view, and it will be easy to outline 

 on the paper the position of the image of the scale on the micrometer. Trace 

 the position of a few of the divisions of the scale. 



4. With a millimetre scale measure the distance between any two of the 

 lines sketched. 



Let n =the distance in millimetres of two adjacent divisions, and M =the 

 magnification of the optical system employed. 



Since the scale on the micrometer slide is divided into T J^ mm., it follows 

 that 



Suppose, for instance, the distance between two adjacent lines on the 

 paper be 5 mm. The magnification produced will be 100 x 5. This is 

 expressed by saying that the magnification is 500, or to be more accurate 

 500 diameters. 



The magnification produced by an optical system can also be determined by 

 simply projecting the magnified divisions of the micrometer directly on to a milli- 

 metre scale arranged on the same level as the microscope stage. 



Then, if n denote the number of divisions on the scale occupied by m divisions 

 on the micrometer, 



the magnification is = 100 . 

 m 



