76 



Place a slide of this upon the stage, and with a low 

 power eye-piece select a diatom which appears to be 

 flat ; such a one may usually be found when there 

 are a number on the slide. After the objective has 

 been attached to the nose-piece, focus carefully and 

 observe whether any lines can be seen ; if not, grasp 

 the milled edge of the adjustment collar between the 

 thumb and first finger of the left hand, keeping the 

 fingers of the right hand upon the micrometer screw, 

 or vice versa, if from the outset it was made a habit 

 to use the left hand on the fine adjustment ; turn the 

 collar slightly toward its open point, and as this will 

 place the object out of focus, move the fine adjust- 

 ment correspondingly ; continue to turn the collar, 

 little by little, and do not cease to observe closely ; 

 also, after each movement, focus above or below the 

 plane of the object, so that this will be indistinct, and 

 look for the lines. . Possibly after a little they will 

 begin to appear faintly ; but, if not, continue to bring 

 the collar toward the middle. The lines must now 

 soon make their appearance, and, when they do, it 

 will probably be above the plane of the diatom. This 

 is an indication that the objective is approaching its 

 correction for the cover. Now keep the lines in focus, 

 while the correction collar is being gradually turned, 

 until the lines and the outline of the diatom lie in one 

 plane; the objective is now said to be corrected for 

 cover. Observe which number corresponds to the 

 index, and note this upon paper ; again return the 

 collar to its closed point and go through the same 



