When the diatoms are resolved in this manner the 

 lines will appear to be diagonal in some, longitudinal 

 or transverse in others, according to their position, and 

 if the resolution is very good, these lines will further 

 resolve themselves into minute beads of a hexagonal 

 form. 



It will now be well to bring the mirror more nearly 

 to a central position. Do this by intervals of about 

 10 degrees and note the appearance at each decrease 

 of obliquity. It will be found that as the mirror 

 approaches the optical axis the lines will appear to 

 become more faint and may disappear before central 

 illumination is reached ; in this case it will be well to 

 begin again. An endeavor should be made to make 

 each attempt give better results than the preceding one. 

 Repeated trials will not only impress the various 

 phenomena upon the mind, but will cause a notable 

 improvement in manipulative skill and thus a better 

 performance in the objective. 



To Judge Spherical Aberration. This is a 

 matter of experience based upon the knowledge of the 

 principles involved and after having been studied will 

 be found to be of the utmost value in utilizing the 

 capacity of a microscope. To judge sperical aberration 

 by the use of histological or biological objects, without 

 a previous knowledge acquired from objects which are 

 more suited, is extremely difficult. One may be aware 



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