igo FOCUS OF OBJECTIVES {CH. VII 



of these complex parts with real accuracy would require an amount of knowl- 

 edge and of apparatus possessed only by an optician or a physicist. The work 

 may be done, however, with sufficient accuracy to supply most of the needs of 

 the working microscopist. The optical law on which the following is based 

 is ; — " The size of object and image varies directly as their distance from the 

 center of the lens." 



By referring to Figs. 14, 16, 26, it will be seen that this law holds good. 

 When one considers compound lens systems the problem becomes involved, • 

 as the center of the lens system is not easily ascertainable hence it is not 

 attempted, and only an approximately accurate result is sought. 



§ 262. Determination of the Equivalent Focus of Ob- 

 jectives. — L,ook into the objective to be tested and locate the posi- 

 tion of the back lens. Indicate this on the outside of the objective 

 mount. This is not usually at the optical center, but a near enough 

 approximation for this experiment. Put the objective in position 

 on a microscope whose draw- tube may be extended 250 mm. i. e.: 

 sufficiently to give a tube-length of 250 mm. If the draw-tube is 

 not of sufficient length put on an extension piece. 



Select a positive ocular. One of the Filar micrometers is very 

 satisfactory (Fig. 119). A Huygenian ocular is not satisfactory 

 for this purpose. Use a stage micrometer as object. With exten- 

 sion piece and draw-tube make the distance between the back lens 

 of the objective and the position of the cross lines of the filar mi- 

 crometer 250 mm. This is so that the image distance shall be 

 250 mm. 



Arrange the filar micrometer so that its movable line shall be 

 parallel with one of the lines of the stage micrometer, and then proceed 

 to measure the space, making several measurements and taking the 

 average as directed in § 190. But in this case it is. necessary to 

 know the size of the real image in millimeters. The pitch of the 

 screw we will suppose is ^mm. as in the one figured (Fig. 119) 

 then the whole revolution will move the traversing line }4 mm., 

 and the partial revolutions may be read on the graduated drum each 

 graduation representing a movement of 0.005 mm - or 5H- Suppose 

 it requires 2.50 revolutoins of the drum to pass the movable line 

 over ^ of a millimeter on the stage micrometer. Then the size 

 of the real image of T : T mm. is two and one-half revolutions multi- 

 plied by the value of one revolution or the pitch of the screw which 

 is one-half of a millimeter thus : 2.50X 0.5 = 1.25 mm. Now if the 

 object is j 1 ^ mm. and the real image is 1.25 mm. the magnifica- 



