308 THE AMERICAN MONTHLY [Oct 



scale should show the white centre to the note of excla- 

 mation as it is called, with a narrow constriction near its 

 broad end, and its point as a fine straight line extending 

 some long way down in the black. Diatoms possessing 

 the finest markings should be now resorted to, comparing 

 the quadrilateral dots in the Surirella gemma, the hex- 

 agonal markings in the Pleurosigma angulatura, the can- 

 aliculum in the Pleurosigma halticum described by Dr. 

 Van Heurck, and the small central portions of the Aula- 

 codiscus sturtii. To these may be added by some au- 

 thorities the resolution of the close rulings in Nobert's 

 lines. When using oblique light with a condenser of 

 high N. A., the lines in Amphipleura pellucida, should 

 stand out well defined, if the specimen be well marked. 

 In all these tests the performance of the apochromatic is 

 unapproachable by the achromatic, especially in the case 

 of photography with them. It must be here remarked 

 that all high-power objectives, according to a great au- 

 thority in lens construction, can be made for special ob- 

 jects to give much more excellent results than those lenses 

 corrected for general purposes, but then the objective is 

 so restricted in its application that it necessitates others 

 being obtained each for its special use, which involves 

 great expense. By this is meant a lens constructed to 

 show bacilli with a flat field may not perform so well for 

 resolving the fine markings on diatoms ; and one spe- 

 cially corrected to separate lines with ''oblique light" 

 may often not be such a good "all-round" lens as another 

 which does not perform under these circumstances quite 

 so well. This constitutes a great difficulty when com- 

 paring the performance of different lenses. When using 

 ordinary achromatics for photography, a great falling-ofF 

 in results is evident at once unless they are specially cor- 

 rected for the purpose. Of late years, however, object- 

 ives have been made which give very fair results as pho- 

 tographers with good performance as visual lenses, and 



