Royal Microscopical Society. 
257 
being minute; or striated.”* In the last edition of ‘Pritchard’s 
Infusoria ’ we are told that Eupodiscus Argus is “easily recognized 
by its irregular cellules and intervening puncta, which give the 
disk a clouded appearance very unlike the usual transparency of 
Diatomacese.” 
Until Mr. Wenham’s researches settled the old disputes f as to 
whether -the markings on ordinary diatoms were elevations or 
depressions such phraseology as the above might be admissible, but 
careful examination of the objects with the best optical means now 
at the command of the microscopist may be expected to banish 
such terms as “ areolar,” “ cellules,” &c., from the descriptions of 
diatoms ; the so-called “ cellules ” being spherules of silica, and 
the “ areolae ” ( little spaces ) — not at all corresponding with what is 
generally meant by such a term. 
At the end of Mr. Wenham’s paper on the new illuminator, he 
remarks that “ in English-mounted slides of diatoms and insect 
scales, some are always found detached from the cover and on the 
slide, and are shown well with it ; but such is not the case with 
Moller’s and others : they raise their covers off the slides by a ring 
of cement, and in many of them, I have searched in vain for a 
solitary specimen that has become detached.” In my specimen of 
Moller’s type slide some diatoms were found that could be well 
exhibited by Mr. Wenham’s plan, though not by the same action 
of his instrument as when dry slides are used. Among these were 
the Eupodisci placed at the corners of each series. This result was 
not, however, arrived at without a good deal of trouble and practice 
in using the apparatus, and microscopists who take it up for the 
first time must not be disappointed if they frequently fail in its 
employment until they have spent a good many evenings in testing 
its action at various points of rotation, with the sub-stage mirror 
throwing up the light, and at various angles. 
When successfully employed upon Eupodiscus Argus a consider- 
able portion of the circular valve will become clear and plain, and 
it will be seen that it is entirely composed of spherules of different 
sizes and varied aggregation. Radiating from a central portion, 
occupied by minute and closely-packed spherules, bands will be 
seen proceeding to the circumference, each one composed of minute 
spherules that appear in close contact under an with eye-pieces 
up to D of Ross’ system. Between these bands are larger spherules, 
frequently, but by no means universally, arranged in fours, so that 
when seen with powers too low for their distinct separation they 
* In the edition of the ‘Mic. Die.’ now issuing in numbers, pi. 12, an 
additional figure is given of part of a valve of E. Argus a little nearer the mark, 
but far from being right. 
f This was written, not anticipating Mr. Stewart’s remarks in discussing this 
paper. The exact refractive power of various diatoms — for they seem to differ— 
should be ascertained. 
