146 



practice with this objective use P. angulatuui. 

 Focus carefully and observe whether any lines can 

 be seen ; if not, grasp the milled edge of the adjust- 

 ment collar between the thumb and first finger of 

 the left hand, keeping the fingers of the right hand 

 upon the micrometer screw. Turn the collar 

 slightly toward its open point and as this will place 

 the object out of focus, move the fine adjustment 

 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 distinct, 

 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 point. 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 objec- 

 tive 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 again return the collar to its closed point and 

 go through the same proceedings as carefully as 

 at first. When the best point is again reached 

 look for the number and see whether it agrees 

 with the first ; very likely it does not, which is 



