442 
Transactions of the Society. 
Photo No. 3 is taken from a valve of Pleurosigma Balticum. In 
this case, as the structure is so very minute, and as I have not been 
able to find even a second specimen presenting the same aspect to 
view, I bring forward an isolated example for what it is worth. As 
however the picture it gives is entirely in accordance with what we 
should expect to see, judging from analogy with the coarser forms 
cited and illustrated above, and about which no doubt can exist, I 
consider it is a substantially true representation, and that in this 
Pleurosigma at any rate the dots represent cavities which extend 
from face to face of the shell. 
The question as to whether these pores are absolutely complete 
perforations through and through the walls of the valve or have any 
sort of operculum or capping on either or on both faces, as is the 
case with the large discoid diatoms, is one which can hardly be decided 
by optical means. But the way in which various precipitates are 
retained in the pores in resistance to violent agitation with water, 
leads me to incline strongly to the opinion that such “cappings” 
exist. Moreover, in the case of one large and coarsely marked form of 
Epithemia the “secondary markings,” i. e. the perforations of the 
cappings, are plainly visible in an imperfectly charged specimen of 
which I now exhibit a photograph (No. 4). 
I must apologize for again trespassing on your attention with such 
a threadbare subject as diatom markings, but plead that it is done in 
the hope of finally laying to rest a subject of controversy which has 
engaged a most disproportionate amount of attention. 
There is indeed still one aspect in which the subject may even yet 
be regarded as of serious interest. So long as the structure of these 
much studied organisms remains in doubt, so long must a great feel- 
ing of uncertainty prevail as to the value to be attached to the inter- 
pretation of appearances presented by other minute structures under 
the Microscope. 
Now, if by any means we attain to a feeling of comparative 
certainty as to the truthfulness of the visual impressions received 
from the Microscope in any one case of minute structure, we pro tanto 
increase our confidence in the reality of the things pictured to us in 
those cases where none but purely optical means are available in 
making out the subject under examination. This method of fill- 
ing the pores of diatoms has rendered it certain that the conclusion 
previously arrived at by observers like Messrs. Flogel, Weiss, Van 
Heurck, H. L. Smith, Cox, Nelson, Morland, and others, viz. that 
the dots, &c., on diatoms were perforations or depressions, and not 
beads, was substantially correct, and that therefore the microscopic 
images on which they founded their opinions were at any rate ap- 
proximately true images. Hence we derive the comforting assurance 
that we need not receive with utter distrust the conclusions arrived at 
by the study of microscopic images, even in those cases where the 
close approximation of minute structures must lead to the production 
of endless diffraction spectra. 
