210 Transactions of the Boyal Microscoincal Society. 



the inch. The original diatom then must have had over 146 trans- 

 verse striae to the xtrW of an English inch, and it is not reflecting 

 on their skiU as microscopists to assert that neither Herr Seibert 

 nor Mr. Hickie could have even glimpsed striae so fine as this, for 

 the objectives which would enable anyone to do so have yet to be 

 constructed. 



It is quite evident, then, that Herr Seibert must have photo- 

 graphed some diatom much larger than FrustuUa Saxonica, and 

 that this larger diatom was a coarse specimen of Navicula Rliom- 

 hoides, I hope to convince those who wiU examine the accompany- 

 ing photographs marked A and B [or their representations in the 

 acco?npanying Plates]. 



On the MoUer's type-plate (specially arranged) belonging to 

 the Army Medical Museum, there is a specimen of Navicula Bhom- 

 hoides • 0069 of an inch long, with 60 " transverse striae " (really 

 rows of beads) to the yuW of an inch. When this is photographed 

 so as to give an image of the same size as the diatom photographed 

 by Herr Seibert, it appears so similar to it in form, and, when the 

 light is suitably managed, in the character and fineness of the 

 striae, as to leave no reasonable doubt that it is of the same species 

 and very nearly of the same size. 



In order to approximate as closely as possible to the conditions 

 indicated by Herr Seibert's photographs, I used an immersion 

 No. 9 of Hartnack, and throwing the light lengthwise to the 

 frustule, obtained the j)hotograph marked A, showing the transverse 

 striae. I intended to make my picture of the frustule the same size 

 as Herr Seibert's ; and, in fact, on my negative it is the same size, 

 as on the print from his in my possession ; but as prints spread 

 more or l^ss when rolled, and shrink sometimes when not rolled, of 

 course the frustule on my negative is not precisely of the same size 

 as on his, though it must be very nearly so. My negative proved 

 to be magnified 580 diameters, and the striae counted 103 to the 

 inch on the negative — (on the paper prints they vary of course). 

 Next, throwing the light transversely to the length of the scale 

 I obtained the photograph marked B, which shows the longitudinal 

 lines quite like those in Herr Seibert's second picture. 



I think no candid observer who compares these pictures with 

 Herr Seibert's, will hesitate to admit that the two diatoms repre- 

 sented are very similar frustules of the same species. The small 

 difference in the number of striae might have been somewhat 

 reduced had I been able to make the diatom in my pictures of 

 exactly the same size as that in Herr Seibert's ; and the resem- 

 blance would have been still further increased if I had stopped out 

 the background with some opaque paint, as he has done. Instead, 

 I preferred to stop it out with tissue paper, which, while gi\'ing 

 prominence to the central diatom, permits all the other objects in 



