148 ELEMENTARY CHEMICAL MICROSCOPY 



Method 3. Measurements calculated from the magnifying 

 power of the microscope. This method is the least satisfactory 

 of those discussed and is rarely employed, but needs considera- 

 tion since it involves a determination of the magnifying powers of 

 the microscope. 



"The magnification of a compound microscope is the ratio 

 between the final and virtual image and the object magnified" 

 (Gage 1 ). Magnification depends upon the power of the objec- 

 tive, that of the ocular and the distance between objective and 

 ocular, and any change in any of these will alter the magnifying 

 power of the instrument. 



Magnifications are always recorded for a distance of 250 milli- 

 meters, the distance of most distinct reading vision of the normal 

 human eye. 



The magnification of a compound microscope is most easily 

 ascertained by holding a piece of ground-glass, tracing paper or 

 tracing cloth at a distance of 250 millimeters from the stage, 

 excluding all side lights with a screen or dark cloth. The pro- 

 jected rulings of a sharply focused stage micrometer are meas- 

 ured with a pair of dividers or a scale. Dividing the size of this 

 image by the actual size of rulings gives the magnification for 

 standard distance. 



Instead of employing a screen as above, we may employ a 

 drawing camera, using as above a stage micrometer and pro- 

 jecting the image upon notebook or drawing board placed at 

 the standard distance of 250 millimeters, measuring from the 

 upper or outer face of the prism of the camera. 



When an Abbe camera lucida is used with the microscope in a 

 vertical position, in order to obtain the standard distance of 

 magnification, one measures from the outer surface of the Abbe 

 prism (this being the limit of the uppermost reflected ray) to the 

 mirror and from the mirror to the drawing paper as indicated 

 in the diagram abc, Fig. 52, page 103. To obtain this distance 

 it is necessary to raise the notebook BB above the plane of the 

 table top. With the paper in this position the magnified image 

 of the stage micrometer rulings is sketched or the intervals taken 



1 The Microscope, loth Ed., 1908, p. 119, Ithaca, N. Y. 



