424 On Surirella gemma as a Test- Object. 



which accompany this memorandum the number was 91 to 

 the -ToVo of an inch. Larger frustules exhibited rather coarser, 

 smaller ones rather finer striee. On the smallest frustules at 

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

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

 7 o'o of <^n inch. The stria? of these smallest and most difficult 

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

 the Nobert'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 slides oi AmpMpleura 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 Century 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 stride count the same number to the 

 - , o'o of ^11 inch. In any event, I have found, as yet, no slides 

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

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

 tules the stria? of which exceeded 100 to the tttott of an inch. 



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

 light was not very satisfactory. 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 Sririrella gemma has been recommended by Hartnack 

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

 gained access to his original description, but find accounts of 

 his views, with figures, in the works of Drs. Carpenter and 

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

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

 tudinal strias 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 



