140 



tudinal 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 at intervals 

 of about 10 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 a 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 

 spherical 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, by the 

 unsatisfactory appearance of the image, that 



