144 



Note on Fine Markings on Navicula Major. 



By Edward M. Nelson, F.R.M.S. 



{Read March \bth, 1895.) 



About twenty years ago I discovered two narrow bands of so- 

 called stria? on the hoop of this diatom, and curiously these were 

 noticed before they were resolved by perceiving their blue sheen, 

 when the diatom was examined on a dark ground with an objective 

 whose aperture was insufficient to resolve it into stria?. 



At that time Messrs. Powell and Lealand had just brought out 

 (December, 1874) their new formula water immersion |, and when 

 that lens was tried on this test the so-called stria? were readily seen ; 

 subsequently they were resolved with a P. and L. dry £ of 90°; 

 the diatom was then kept for many years as a test for lenses of 

 about that aperture ; afterwards the diatom yielded to the same £ 

 with central, and to a P. and L. T 4 F of 80°, with oblique light. 



Coming to more recent times, Zeiss' apo. \ of "65 N.A. resolves 

 it with a -| cone of central light. These stria? will be found to be 

 fairly constant at 60,000 per inch ; they therefore resemble the so- 

 called transverse stria? on a Cherryfield Rhomboides. Arguing 

 by analogy we may conclude that these stria? are composed of rows 

 of closely adjacent minute apertures ; therefore for some time past 

 this diatom has been examined with a view to the resolution of 

 these smaller details when any new instrumental improvement 

 appeared. 



When the fluorite condenser was fitted with a correctional 

 collar, this object was the first it was tried upon, and after a little 

 perseverance the structure was at last fully resolved by the Zeiss 

 apo. -I- of 1*4 N.A., using a| cone of central light. 



You will notice that this diatom affords an exceedingly good 

 test, because there is no rapha? present to yield false ghosts, and 

 also a marked difference will be observed between this and other 

 similar diatomic structures, inasmuch as the minute apertures are 

 not in rows,* so there are no so-called longitudinal stria?. On this 



* A rough sketch will be found in the " R.M.S. Journal " for 1895, page 

 231. (Notice the square appearance of the holes.) 



