CHAPTER VII. 



DETERMINATION OF MAGNIFICATION. MICROMETRIC 

 MICROSCOPES— MICROMETRY. 



Determination of Magnification. — It not infrequently hap- 

 pens that the determination of the magnification of a certain 

 combination of eyepiece and objective is of considerable impor- 

 tance and that in the table of magnifications listed by the maker 

 of the instrument this particular combination is not given; 

 moreover it is customary to indicate upon all drawings and photo- 

 micrographs the number of times, in linear dimensions, the speci- 

 men has been magnified. It has become the custom to indicate 

 the magnification thus: X 150, meaning the drawing is 150 times 

 the size of the object. 



" The magnification of a compound microscope is the ratio 

 between the final or virtual image and the object magnified." * 



Any changes in the instrument which will cause a change 

 in this ratio will be followed by a change in magnification. The 

 usual changes practiced are; (a) changes in eyepiece or objec- 

 tive, (b) lengthening or shortening the draw-tube, (c) increas- 

 ing or decreasing the distance at which the virtual image is 

 viewed, as for example upon a ground-glass screen. 



The approximate magnification of the microscope may be 

 ascertained by multiplying the initial magnification of the 

 objective (see Chapter I) by the magnification of the eyepiece. 



The Determination of Magnification in a compound micro- 

 scope is most easily accomplished by holding a piece of ground- 

 glass, tracing paper or tracing cloth at a distance of 250 milli- 

 meters from the stage, excluding all side light with a screen of 

 dark cloth. The image of the rulings of a sharply focused 

 stage micrometer projected upon the ground-glass are measured 

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



1 Gage. The Microscope. 12th Ed., p. 135, Ithaca, N. Y., 1917. 



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