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NITZSCHIA SINGALENSIS ! A NOTE ON MR. 

 MERLIN'S PAPER. 



By M. a. Ainslie, R.N., B.A., F.R.A.S. 



(Read April 25th, 1916.) 



It has long been recognised that as a test for resolution, at any 

 rate with oblique light, our old friend Amphipleura 'pellucida 

 leaves something to be desired ; as Mr. Merlin says, the exhibition 

 of its transverse striae is no great feat at the present day. 



I am inclined, however, to think that no resolution, even of 

 the most difficult diatom, when performed by means of oblique 

 light, affords any real criterion of the excellence of an objective ; 

 indeed, I have heard it whispered that at one time it was not an 

 uncommon practice with some opticians to correct their immersion 

 objectives to give the best possible resolution with oblique light, 

 without much attention being paid to the correction of the central 

 and intermediate zones, which would be more useful for general 

 purposes. So long as their customers were desirous of possessing 

 objectives "which will resolve A. pellucida,^^ and regarded the 

 performance of this feat as the principal duty of an immersion 

 objective, it was natural that opticians should make every effort 

 to gratify their wishes. I have myself obtained excellent resolu- 

 tion of the transverse striae, at any rate, of A. fellucida, with 

 objectives which were practically useless for ordinary purposes, 

 such as the examination of Bacteria, etc. 



Something better is required at the present time ; and until we 

 have, as is much to be desired, specimens of N, singalensis 

 mounted in realgar, I am inclined to think that the resolution of 

 the transverse striae of A. pellucida in this medium, with a full 

 solid axial cone of illumination, remains the best existing test of 

 the all-round capacities of an oil-immersion objective ; at any 

 rate, in the course of the e.xamjnation of a great nujnber of 



