A. A. MERLIN ON RESOLUTION OF AMPHIPLEURA PELLUCID A, ETC. 5 



Heurck gives 30 striae in 0-01 mm. for the G. oceanica var. 

 indica Grim., and 30 to 31 for the G. oceanica var. novae- 

 zeelandiae Grun. Some specimens of G. suhtilissima, however, 

 are finer, running at about 88,000 to the inch. 



Navicida crassinervis. Striae 34 to 35 in O'Ol mm. according 

 to Yan Heurck. This has proved a most delicate object with 

 the 4 mm., both dry and in realgar. With N.A. r3 and 

 1-4 realgar mounted valves are sharply resolved into dots, but 

 the transverse striae have alone been seen with the dry lens. 



Hyalodiscus subtilis. In a mixture of monobromide of 

 naphthaline and balsam. Dotted structure on outer zone well 

 seen, although faint and difficult near the edge of the disc. In 

 balsam mounts the structure appears still fainter, but never- 

 theless may be traced nearly to the outer edge, where it runs 

 at about 76,000 to the inch. 



Surirella gemma Ehrbg. In realgar the beading has been 

 seen beautifully defined with the valve arranged longitudinally 

 on the sharply focussed edge of the lamp flame. Specimens 

 mounted dry, in balsam, and in quinidine, have been also 

 examined, but their complete resolution has proved a much more 

 difficult matter. 



Colletonema vulgare. Moller's balsam type slide. This has 

 been most carefully studied with the 4 mm. The resolution 

 is very faint, and requires particularly exact focal adjustment, 

 but when once seen it can be held fairly steadily without any 

 great difficulty. Dr. Yan Heurck writes of this diatom, " Stries 

 fines, delicates, les moyennes faiblement radiantes, les terminales 

 paralleles, environ 34 en 1 c.d.m. ; les stries medianes plus fortes, 

 plus ecartees, 24 en 1 c.d.m. et plus radiantes." 



Navicula major. Moller's balsam type slide. The full reso- 

 lution of the structure of the bands on the hoop of this diatom 

 is by no means easy, even with the Zeiss 3 mm. apochromat of 

 N.A. 1'4. Notwithstanding this, the resolution is carried very 

 far by the 4 mm., the striae appearing remarkably black, crisply 



