90 



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 such a 

 point where the definition of the object shall 

 appear impaired without regard to the illumina- 

 tion of the field. These limits will mark the 

 efficient beam of light which passes through the 

 rear system of the objective. As we have stated 

 before, in some objectives the rear system is larger 

 in diameter than the effective beam of light trans- 

 mitted 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. The following is called 

 Lister's method. 



After the objective has been focused, place the 

 body of the microscope 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. Indi- 

 cate the position of the center of the lamp at each 



