4 Transactions of the Royal Microscopical Society. 
battery of lenses became improved, I was enabled to resolve into 
hemispheres all the larger, and most of the medium-sized ones, and 
eventually to consider myself master of the resolution of this 
diatom ; a work which I accomplished not for its own sake, but 
merely to have a given set of marked tests, of known value to 
myself, for the sake of future comparison. When therefore the 
value of “ F. Saxonica,” as a test for high powers, was announced, 
I proceeded at once to procure it. My first specimens were got 
from London ; but to my surprise the forms sent under this name 
were neither more nor less than the medium-sized frustules of my 
now well-worked specimens of N. rhomboides. I of course con- 
cluded that there was an error in naming, and at once wrote off to 
two other sources for the form I required. ‘To my great vexation, 
these were merely repetitions of what I had received before, one of 
them containing really coarser specimens than any I could find on 
Mr. Hardman’s slides of N. rhomboides. Determined to obtain the 
right frustules, I wrote to an English friend, then in Germany, — 
and was favoured with specimens, slightly smaller, but in every 
sense identical with those supplied from English sources. I failed 
to discover a single difference between them. I could striate, or 
resolve them into dots, precisely as I did my N. rhomboides. Their 
midribs—a very characteristic feature—were precisely like Rhomb- 
oides, and their general contour varied, as N. rhomboides varies, 
some frustules having an inclination to an obtuse angle in the 
centre of the margin, while others preserve in their margin a 
continuous curve. 
I had never made the natural history of diatoms a study; so 
the inference I was obliged to make was, that there must be some 
morphological or developmental difference between these diatoms 
which justified the difference of name: but as tests I determined 
that there was certainly no difference between them. 
When I first obtained N. crassinervis, my difficulty was almost 
as great; for although the frustules on the slides so named were 
certainly much smaller than the majority of frustules on my speci- 
mens of Lhomboides, still I could find some almost, if not quite, as 
small; and I could detect, in an examination of a number of 
specimens, nothing that had not its complete equivalent in the 
frustules of Rhomboides. I felt therefore bound to conclude that, 
as test-objects, these three separately named diatoms were merely 
Rhomboides of various sizes, and therefore, generally speaking, of 
greater or less difficulty in “ resolution.” I reached this conclusion 
nearly three years since, but as it was an opinion formed on the 
silicious frustules alone, and had no connection with the natural 
history of the form, I merely set it aside for what I considered it 
worth. But as competent authorities have now—fully knowing 
the nature of the plant—determined that the three forms in 
