174 Transactions of the 



preparations made on balsam and without it, I observed that in the 

 first case the striiB, or better, the longitudinal divisions of S. gemma, 

 when observed in an oblique, monochromatic light, present a 

 •wicker-basket texture, and make us believe that Hartnack was 

 right when he interpreted and sketched them like oblong hexagons. 

 But in the second case in which, for Moller's test-plate prepared on 

 balsam, I substituted dry shells, simply covered with a thin glass, 

 they presented immediately nice moniliform striae, or rows of small 

 spheroids, so disposed, that the polar axis of each of them was equal 

 to f^rds of the equatorial. I may add, moreover, that, although the 

 difficulty of distinguishing the most minute particularities of a 

 diatom prepared on balsam be far greater than that of seeing it in 

 its dry condition, I did even succeed, after a little pain and trouble, 

 in perfectly making out the spheroids in Moller's own preparation. 

 And more than that, under a j)roperly arranged monochromatic 

 light, their appearance did turn out so evidently clear, as to leave 

 not the least doubt or perplexity about the right interpretation of 

 such wonderfully minute details. This I could not see perfectly 

 till late, except with 'a good No. 10, of Hartnack; but this year 

 I obtained the very same result with another No. 10, which Mr. 

 Nachet was kind enough to construct purposely for my trial. 



The nature and form of the structural details of that interesting 

 diatom being thus set aright, it is plain at the same time that Mr. 

 Bicknell was not the only one who could witness the structural 

 features of the Surirella in question. His perception, besides, 

 having been not quite true to the fact, though his shortcoming was 

 very excusable, as some of the most distinguished micrographers 

 were liable to the same mistake for several years, taking for hexa- 

 gonal the spheric markings of Pleurosigma angulatum. I had 

 thought so myself at first, deceived by imperfect photographs, and 

 backed, as I was, by the authority of first-rate micrographers who 

 had preceded me in such trials. But I was soon put right, first by 

 better photographs, which I obtained more clear and accurate with 

 the exquisite F objective of C. Zeiss, of Jena ; and then again by 

 wonderfully increasing powers which Colonel Woodward was Idnd 

 enough to send me, for which I publicly express my gratitude, 

 waiting the first opportunity to return it by some of the best photo- 

 graphs of my own collection. 



Thus my successful studies were happily performed by means 

 of Hartnack and Nachet's No. 10. Yet whoever does make use of 

 these valuable objectives with a simply white light cannot expect to 

 obtain more than the mentioned wicker-basket appearance, not the 

 precisely moniliform structure of Surirella gemma. More than 

 once Hartnack himself showed me the said diatom under this sort 

 of light, not only with his No. 10, but even with Nos. 9 and 8, 

 holding as his private opinion that with his 7 merely he might have 



