94 MAGNIFICATION AND MICROMETRY. [CH. IV. 



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 magnification must 

 bei5-i-2 = 7^3. That is, the image is 7^2 times as long or wide as 

 the object. In this case the image is said to be magnified 7^ diame- 

 ters, or 7)^ times linear. 



The magnification of any simple magnifier may be determined exper- 

 imentally in the way described for the tripod. 



MAGNIFICATION OF A COMPOUND MICROSCOPE. 



§ 149. The Magnification of a Compound Microscope is the 

 ratio between the final or virtual image (Fig. 21, B 3 A 3 ), and the object 

 magnified (A B). 



The determination of the magnification of a compound microscope may 

 be made as with a simple microscope (§ 147), but this is very fatiguing 

 and unsatisfactory. 



§ 150. Stage, Object or Objective Micrometer. — For determin- 

 ing the magnification of a compound microscope and for the purposes 

 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 on glass is most convenient. The spaces 

 between the lines should be T V and T \-§ millimeter, and when high pow- 

 ers are to be used the lines should be very fin 2. It is of advantage to 

 have the coarser lines filled with graphite (plumbago), especially when 



low powers are to be used. If one has 



an uncovered micrometer the lines may 



be very readily filled by rubbing some of 



the plumbago on the surface with the end 



of a cork ; the superfluous plumbago may 



Fig. 91. Diagram of a stage be removed by using a clean dry cloth or 



micrometer, with a ring on the a p j ece f i ens paper . After the lines are 



lines to facilitate finding them. m ^ ^ the plumbago wiped from the 



surface, the slide should be examined and if it is found satisfactory, i. e. , 

 if the lines are black, a cover-glass on which is a drop of warm balsam 

 may be put over the lines to protect them. 



If one desires to have a part of the micrometer uncovered and a part 

 covered for using homogeneous objectives, the lines may be filled with 

 fine graphite, as described, and a piece of oblong cover-glass placed over 

 a part of the band of lines. 



§ 151. Determination of Magnification.— This is most readily ac- 

 complished by the use of some form of camera lucida (Ch. V), that of 



