OBJECT GLASSES. 97 



tract one reading from the other, and the difference is 

 the angle of aperture. 



There being no means for the actual determination of 

 the bisection of the fields, this having to be judged of 

 by estimation, follows that the plan is not rigidly 

 accurate; the author has, however, measured repeatedly 

 the same object-glass, following as above described, and 

 without a variation in the results of more than a half 

 degree; many of his pupils, too, on their very first 

 attempts, are quite as successful. 



The bisection of the fields is much more easily arrived 

 at in the evening. 



OBJECT-GLASSES. 



Years ago wrote Dr. Carpenter thus: 



" It may be safely affirmed that the most perfect 

 object-glass is that which combines all the preceding 

 attributes (viz., defining power, penetrating power, or 

 focal depth, resolving power, and flatness of field) in 

 the highest degree in which they are compatible one 

 with another. But, as has just been shown, two of the 

 most important viz., penetrating power, and resolv- 

 ing power stand in such opposite relations to the 

 angular aperture, that the highest degree of which each 

 is in itself capable can only be obtained by some sacrifice 

 of the other ; and therefore cf two objectives which are 

 respectively characterized by the predominance of those 

 opposite qualities, one or the other will be preferred by 

 the microscopist, according to the particular class of 

 researches he may be carrying on ; just as a man who is 



7 Microscopy. 



