424 On Sunrella gemma as a Test- Object. 



which accompany this memorandum the number was 91 to 

 the TiTo-o of ^11 inch. Larger frustules exhibited ratlier coarser, 

 smaller ones rather finer stria3. On the smallest frustules at 

 my disposal (several of them only rrh-o i^^ch in length) I found 

 no example in which the number of stria exceeded 100 to the 

 -j-oVo of ail inch. The strije of these smallest and most difficult 

 frustules do not, then, rival in fineness the nineteenth band of 

 the Robert's plate, as has been asserted by some ; they com- 

 pare rather with the sixteenth and seventeenth bands. 



After making the photographs, I extended my observations to 

 a number of other sHdes oi AmjjMpleura pellucida — including 

 two of the original specimens from Hull, kindly sent to the 

 museum some time since by Mr.W. S. Sullivant, of Columbus, 

 Ohio, and the example in the First Centmy of Eulenstein. I 

 found that different slides varied considerably in the ease with 

 which I could resolve them, chiefly, as I think, on account of 

 the thickness of the glass covers, which in several instances 

 did not permit the best work of the immersion -^. Perhaps, 

 however, the markings on some frustules may be shallower 

 than on others whose striee count the same number to the 

 1 o'oo of an inch. In any event, I have found, as yet, no slides 

 the covers of which permit the -^ to be approximately adjusted, 

 on which it was impossiljle to resolve the frustules, and no fi-us- 

 tules the stria of which exceeded 100 to the to^-o of ^^ inch. 



The best resolution I was able to obtain by ordinary lamp- 

 light was not very satisfactoiy. I used, therefore, during the 

 investigation, direct sunlight rendered monochromatic by pas- 

 sage through the solution of ammonio-sulphate of copper, A 

 parallel pencil of such light was concentrated by the achro- 

 matic condenser, which was suitably decentred to obtain ob- 

 liquity. The same illumination was employed in making the 

 photographs. I have since had the pleasure of exhibiting the 

 resolution in quite as satisfactory a manner to several micro- 

 scopists by monochromatic light obtained from the electric 

 lamp. 



The HurireUa gemma has been recommended by Hartnack 

 as a test for immersion-objectives of high powers. I have not 

 gained access to his original description, but find accounts of 

 his views, with figui-es, in the works of Drs. Cai-penter and 

 Frey (The Microscope and its Revelations, 4th ed. p. 182 ; 

 Das Mikroskop, 3rd ed. p. 40). Hartnack observed fine longi- 

 tudinal stria in addition to the fine transverse ones previously 

 known to exist between the large transverse ribs ; he sup- 

 posed the true markings to have the form of elongated hexa- 

 gons. 



Two handsome slides of this diatom were received at the 



