Ill 



ON NITZSCHIA SINGALENSIS AS A TEST-OBJECT 

 FOR THE HIGHEST POWERS. 



By a. a. C. Eliot Merlin, F.R.M.S. 



{Read April 25th, 1916.) 



Plate 8. 



I AM indebted to the kindness of my friend Mr. E. M. Nelson for 

 a styrax mount of this very interesting diatom from Amherst, 

 Burma. Its minute structure is undoubtedly of a similar nature 

 to that of other and coarser varieties of the genus, but although 

 the form is large, the structure is so fine that hitherto only the 

 transverse striae have been seen by their discoverer, Mr. Nelson, 

 and up to the present most other microscopists have failed to 

 see them. Under these circumstances the photograph (PI. 8) 

 taken with a l/8th apochromat of 1*42 N.A. at a magnification of 

 3,600 diameters by means of oblique sunlight from a heliostat, 

 which clearly reveals the transverse striation, will, I venture to 

 think, prove of interest to many members of our Club, which 

 has done so much in the past in an unpretentious way to en- 

 courage advancement in all matters appertaining to the micro- 

 scope. 



At present many of us must feel that our old friend Amphi- 

 fleura pellucida can no longer claim the proud position which it 

 so long held as the test for objectives of the highest power. As a 

 matter of fact, for many years past, any tolerable oil-immersion 

 lens of moderate aperture could be relied upon to reveal easily 

 its once elusive transverse striae (which run at from 93,000 to 

 100,000 to the inch) by means of oblique light. Keally good 

 objectives of 1*3 N.A. will reveal them in balsam without any 

 great difficulty with axial illumination, and I have photographed 

 clearly striae at 95,000 to the inch in a realgar preparation of 

 A. pellucida with central light from the edge of the lamp flame. 



