2'JO correspondence. 



him to speak out plainly ; and if he says he has, I am prepared 

 beforehand to disbelieve him." 



I have no right to assiune — what certainly does not appear from 

 Mr. Singleton's letter — that he has given any special attention to 

 diatoms as tests ; but those who have done so know well that, when 

 once they have fully succeeded in mastering such a test with a 

 moderate power — say a ^V ii^ch — a little practice and perseverance 

 soon enable them to overcome it with a weaker objective, and that a 

 little further practice generally resolves it with a weaker power still. 

 Resolution turns not so much upon objectives as upon the manipu- 

 lator's own fingers. 



My use of the words " up to S. gemma " was an intentional con- 

 cession to the frailty of human nature, in order not to oifend the 

 prejudices of those who persist in classing that diatom as a very high 

 test. 



In my own practice, I know only four first-class tests : (1) Amphi- 

 pleura pelliicida, (2) Sfaiironeis spicula, (3) Navicula crassinervis, 

 (4) Frustulia Saxonica, as it is found in Eastern Prussia. In some 

 of these latter, especially in those mounted by C. Rodig, of Hamburg, 

 the lines are so amazingly fine, as to render this kind of Frustulia a 

 more difficult test even than the acus. The second mentioned may be 

 found on almost any slide of P. macrum. It is a small lanceolate 

 species of Stauroneis, somewhat like two dagger-blades placed hilt to 

 hilt. I recommend it to the notice of the readers of this Journal. 



But with regard to S. gemma itself, I attach no sort of value to 

 this diatom ; and I have often wondered how such a thing ever got 

 voted into a test. 



Its deficiencies are obvious. It almost never presents an even 

 surface ; there is no uniformity in the same gathering in respect of 

 difficulty, some specimens, especially those of a whitey-brown colour 

 with almost black ribs, being troublesome enough, while many of 

 those with a greenish tinge are scarcely a test for an ordinary ^ inch. 

 For instance, I have one such slide so easy of resolution, that I can 

 resolve six out of every seven of those that lie perpendicularly, 

 taking them just as they come. In some the interspaces between the 

 ribs are twice or thi-ee times as wide at one end as they are at the 

 other. In others, again, some of the ribs run only half-way, leaving 

 a wide space ; and though the longitudinal lines almost invariably, 

 instead of forming straight vertical lines, slope somewhat from right 

 to left, I have sometimes met with instances to the contrary. 



In short, S. gemma seems made for the express purpose of 

 upsetting all our theories as to the uniformity of structure and 

 regularity of marking of Diatomacefe, and, like many Christian men 

 and women, is consistent only in its inconsistency. I think, therefore, 

 I am justified in discarding it as a standard test. 



Now, if I can produce satisfactory evidence that I have shown, 

 clearly and distinctly, the longitudinal lines with an antiquated 

 \ inch, it will, I suppose, render it somewhat probable that I might 

 " detect " them with a remarkably good \ inch of latest date, espe- 

 cially if the latter have the advantage of illumination so helpful as, 



