66 THE MICROSCOPE. 



the object-glass will be employed to adjust for charac- 

 ter of definition, and the fine screw movement of the 

 microscope for correct focus. 



The graduations on the correction-collar are merely 

 for convenience of registering the point of " best cor- 

 rection " for particular objects, so that the objective 

 may be set at the same correction if the observation 

 has to be repeated. It is usual with amateurs, who 

 have not practised themselves thoroughly in rapidly 

 adjusting their objectives by inspection of the image, 

 to note on their slides the best point of adjustment as 

 well as the position of the object, either with reference 

 to stage graduations or to Maltwood's finder. The 

 registration of the position may save time in re- 

 peating an observation ; but the registration of the 

 best point of adjustment should, generally speaking, 

 be regarded as an approximative process only, for the 

 adjusting collar is seldom made so accurately that 

 absolute reliance can be placed on the index. To 

 obtain fine definition test the correction in both direc- 

 tions, and take care to follow the image with the fine 

 adjustment. With objectives of large aperture this 

 process is of much importance, as the exact " distanc- 

 ing " makes or mars the definition. 



Mr. Wenham recommends the following method of 

 securing the most efficient performance of an object- 

 glass. Select any dark speck or opaque portion of 

 the object, and bring the outline into perfect focus ; 

 then lay the finger on the milled head of the fine ad- 

 justment, and move it briskly backwards and forwards 

 in both directions from the first position. Observe the 

 expansion of the dark outline of the object, both when 

 within and when without- the focus. If the greater 

 expansion, or coma, be when the object is without the 

 focus, or farther from the objective, the lenses must 

 be placed farther asunder, or towards the mark " un- 

 covered." If the greater coma be when the object is 

 within the focus, or nearer to the objective, the lenses 

 must be brought closer together, or towards the mark 

 "covered." When the object-glass is in proper ad- 

 justment, the expansion of the outline is exactly the 



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