CH. II 7 } MACXIF1CATION AND M1CROMETRY 119 



The magnification of any simple magnifier may be determined 

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

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

 he has but one eye then the magnification ma}' be determined by 

 the aid of a camera lucida ( 176) or the eikouometer ( 196). 



MAGNIFICATION OF A COMPOUND MICROSCOPE 



174. The Magnification of a Compound Microscope is 

 the ratio between the final or virtual image (Fig. 26, B 3 A 3 ), and 

 the object magnified (A B). 



The determination of the magnification of a compound micro- 

 scope may be made as with a simple microscope ( 172), but this is 

 fatiguing and unsatisfactory. 



175. Stage, Object or Objective Micrometer. For de- 

 termining the magnification of a compound microscope and for the 

 purpose of micrometry, 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 

 ( i X 3 in. , 25 x 76mm. ) is most convenient. 



The spaces between the lines should be y 1 ^ and y^ mm. (or if 

 in inches, y^ and TTJ Vj7 in.). Micrometers are sometimes ruled on 

 the slide, but more satisfactorily on a cover-glass of known thick- 

 ness, preferably 0.15 o. 18 mm. The covers should be perfectly 

 clean before the ruling, and afterwards simply dusted off with a 

 camel's hair duster, and then mounted, lines downward over a shel- 

 lac or other good cell. (See Ch. VII.) If one rubs the lines the 

 edges of the furrow made by the diamond are liable to be rounded 

 and the sharpness of the micrometer is lost. If the lines are on the 

 slide and uncovered one cannot use the micrometer with an oil im- 

 mersion, as the oil obliterates the lines. Cleaning the slide makes 

 the lines less sharp as stated. If the lines are coarse, it is an 

 advantage to fill them with plumbago. This may be done with 

 some very fine plumbago on the end of a soft cork, or by using a 

 soft lead pencil. Lines properly filled may be covered with balsam 

 and a cover-glass as in ordinary balsam mounting (Ch. VII). 



1-6. Determination of Magnification. This is most 

 readily accomplished by the use of some form of camera lucida 

 (Ch. V), that of Wollaston being most convenient as it may be 



