1898.] MICROSCOPICAL JOURNAL. 123 



resolution, which most closely parallels the coarser Lind- 

 heij/ierii valve, may be tlie truest resolution yet attained. 



No doubt tlie above lens was an almost phenomenal 

 one. Every practical raicroscopist knows that the "sim- 

 ilar" objectives, by even the very best makers, are not 

 " all alike," whatever the makers may affirm. They differ 

 in features as in a case above mentioned; most of all in 

 the cone they can employ in critical work, and in what 

 such a cone will reveal. Everyone engaged in difficult 

 research has some favorite objective, treasured and spared 

 in work as much as possible; because he knows full well 

 that if parted with or injured, though he can buy a "sim- 

 ilar" one at the list price, it may be long ere he finds 

 such another. 



22. The question of how far we may still expect ad- 

 vances in our optical powers of reseach is important ; and 

 it is answered very differently according to the " spec- 

 trum " theory, or the qualified views here maintained. 

 It not only follows from the foregoing, but has been over 

 and over again stated expressly by the Abbe school, that 

 We have no hope of further advance, except through in- 

 crease of aperture ; and on that ground was constructed 

 the lens of 1*63 N. A. to be used with flint-glass mounts 

 and dense fluid media — conditions under which it is prac- 

 tically useless. So little are other conditions recognized, 

 that Dr. Van Heurck has only used the chromatic con- 

 denser in his skillful published diatom photographs; and 

 those results are simply 7iil, not one of them surpassing, 

 or in some respects even equalling, what has been done 

 in England with 140 lenses. 



It is far different if the Abbe theory be relegated to 

 its proper place and proportion. Then such " lucky " ob- 

 jectives as the above assume a very marked significance, 

 and hold out a world of promise : in them and in what 

 they tell us lies the future of microscopy. Not the best 

 even of them is probably perfectly corrected for all its 



