In objectives of older construction and in some 

 produced at the present day, the figures are abitrary 

 and serve no other purpose than an index for reference. 



Close the adjustment before attaching the objective 

 as its front may otherwise come in contact with the 

 cover before the focus is reached. For practice with 

 this objective use P. angulatum. 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 milled head of the 

 fine adjustment ; 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 appear- 

 ance, 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 the cover used. Observe 



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