position, remove the mirror with its bow from the 

 socket and place therein, or by rubber band attach 

 to the end of the bar, a toy candle at such height 

 that the flame will be in the optical axis. The 

 mirror-bar is now swung to the right and left to 

 such an extent that one-half the field shall be 

 illuminated, provided the object shall still be well 

 defined, or to the point where the definition of the 

 object shall appear impaired without regard to the 

 illumination of the field. These limits will mark the 

 efficient beam of light which passes through the rear 

 system of the objective. In some objectives the rear 

 system is larger in diameter than the effective beam of 

 light transmitted from the object, which will permit 

 stray rays to reach the eyepiece, but which have no 

 value in forming the image and therefore are not to be 

 considered. In instruments which have no graduated 

 mirror-bar the matter becomes more difficult and less 

 accurate. 



Another method is that of Lister. After the object 

 has been focused upon, place the body of the micro- 

 scope in a horizontal position and in front and some 

 distance from it, a candle or lamp ; if the latter, with 

 the narrow edge of the wick toward the instrument, but 

 level with the tube. Move the lamp on each side of 

 the axis to a point as described in the foregoing 

 method. Indicate the position of the center of the 

 lamp at each extreme on the table and beneath the 



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