Ch. V] 



MAGNIFICATION BY THE M ICROSCOPK 



135 



the virtual image (fig. 77), and as the size of the object is known, the 

 magnirication is determined by dividing the size of the image by the 

 size of the object. Thus, suppose the distance between the two lines 

 at the limits of the image is measured by the dividers and found on 

 the steel scale to be 15 millimeters, and the actual size of the space 

 between the two lines of the object is 2 millimeters, then the mag- 

 nification is 15 -r- 2 = 7.5; that is, the image is 7.5 times as long or 

 wide as the object. In this case the image is said to be magnified 7.5 

 diameters, or 7.5 times linear. 



The magnification of any simple magnifier may be determined 

 experimentally in the way described for the tripod magnifier; but this 



Fig. 80. Stage Micrometer Ruled on a Cover-glass. 



The tenths millimeter (0.1 mm.) spaces are divided by short lines making 

 the whole micrometer one with 0.1, 0.05, and 0.01 millimeters. 



method is of course only possible when the observer has two good eyes. 

 If he has but one eye, or his eyes are very unlike, then the magnifica- 

 tion can be determined with one eye by using a camera lucida or the 

 eikonometer (§ 234, 253). 



§ 232. The magnification of a compound microscope is the ratio 

 between the final or virtual image and the object magnified. 



The determination of the magnification of a compound microscope 

 may be made as with a simple microscope (§ 230), but this is fatiguing 

 and unsatisfactory. 



§ 233. Stage, or object micrometer. — For determining the mag- 

 nification of a compound microscope and for the purposes of microm- 

 etry, it is necessary to have a finely divided scale or rule on glass or on 

 metal. Such a finely divided scale is called a micrometer, and for 

 ordinary work one mounted on a glass slide (1 X 3 in., 25 x 76 mm.) 

 is most convenient. 



