114 M. A. AINSLIE ON NIT ZSCHIA SINGALENSIS I 



different objectives by various makers, I have always found that 

 a good, clear exhibition of these striae under the conditions 

 indicated was a sufficient guarantee of the excellence of the 

 objective for all-round work. 



I do not forget, of course, that Mr. Merlin records that he can 

 see the transverse striae, both of N. singalensis and of A. fellucida, 

 with axial illumination ; in the latter case when mounted in 

 balsam. This appears to me to prove that Mr. Merlin is blessed 

 with a keenness of eyesight far above the average ; I have looked 

 in vain for the slightest trace of the striae under these conditions, 

 and even in the case of A. fellucida the utmost I have been able 

 to do with axial illumination is to see the transverse striae faintly, 

 but certainly, in monobromide of naphthalin, with an apochromat 

 of N.A. 1*40, and with a fluorite objective of N.A. r34. I do 

 not, of course, claim that my own eyesight is in any way re- 

 markable ; indeed, in the matter of the detection of fine dark 

 lines on a bright background — which is here in question — I have 

 reason to believe that my sight is not particularly keen. I am 

 strongly of opinion, however, that this test is, for the great 

 majority of observers, much too severe, and probably too severe 

 also for the great majority of objectives. 



Mr. Merlin's opinion as to the resolving power of objectives with 

 axial light as compared with oblique is certainly in accordance 

 with theory ; but even with the very best objectives obtainable 

 I have frequently found that I could, with oblique light, detect 

 striae which were wholly invisible with axial illumination. 



There is one point in Mr. Merlin's paper which leaves me a little 

 puzzled : he refers to the difficulty he experienced in " stepping " 

 the stri^.e on N. singalensis, owing to their faintness, while on 

 the other hand he sees them with axial light. As far as my 

 experience goes, if diatom striae are visible at all with axial light 

 there is never any great difficulty in measuring them with oblique ; 

 I have measured the striae on A. fellucida in balsam, although in 

 this medium they are, to me, wholly invisible with axial light, 

 without any special difficulty due to their faintness. 



I am showing the transverse striae on N. singalensis, resolved 

 with a Leitz 1/lOth oil-immersion of N.A. (measured) r33, not only 

 because they are of interest in themselves, in connection with 

 Mr. Merlin's valuable paper, but because it so happens that they 

 afford an excellent example of the occasional utility of a device 



