120 



PRACTICAL MICROSCOPY. 



following is a list of the diatoms on this " test-platte," 

 with the number of striations to the inch, as given by 

 Morley : 



Diatom. 



Number of Striae to the Inch. 



It has already been mentioned that the resolving 

 power of objectives depends entirely upon their aperture, 

 with the excellence of their corrections for colour and 

 sphericity. 



Objectives of low angle are generally made with posterior 

 lenses of such a size as to exclude the extreme uncorrected 

 marginal rays, or if this is not done the margins are cut off 

 by a diaphragm. The utilised portion is very fairly 

 corrected for colour, but nevertheless the spherical aberra- 

 tions, though small, must exist ; on the other hand, wide 

 apertures require large back lenses, the rays from which 

 cannot be cut off by a diaphragm without reducing the 

 angle, and therefore the corrections have to be applied to 

 the more oblique pencils ; and chromatic aberration can 

 scarcely be corrected without at the same time affecting 

 the spherical in a very great degree. 



