é THE AMERICAN MONTHLY (Jan 
not this point to the probability of the correctness of my 
suggestion—viz. that the perforations of this diatom 
were already filled up? 
Again, the difficulty in getting an image of a “postage 
stamp” fracture through the perforations of this diatom 
was thought by Mr. T. F. Smith to be due to the exist- 
ence of a layer of beaded structure, which would present 
the appearance of circles bulging outwards; but I imag- 
ine that the clinging of any. silting-up material to the 
broken-into openings would account satisfactorily for 
such appearances. 
The minute appearances which Mr. Nelson discovered 
within the black dots of this diatom may be considered, 
not as pieces of silex or other material, but as irregular 
openings through the silted-up perforations. They are 
bright at the upper focus and dark at the lower-focus 
when an axial cone is employed; that is to say, they be- 
have as do the imagines of ordinary diatomic perforations. 
Pleurosigma formosum is either optically or structur- 
ally peculiar, I cannot imagine the former, so I suggest 
the latter. 
The possibility of the former would raise doubts as to 
whether the microscope were at alla reliable instrument; 
but my suggestion that P. formosum has its perforations 
silted up explains all former difficulties in the interpre- 
tation of the images of this diatom, and, besides, makes 
the exception confirm a method for the interpretation of 
such images as we are considering. And what is that 
method? It is a comparison of the relative positions and 
appearances of the images of unknown structures with 
those of the images of known structures. 
For instance, assume the under-corrected objective and 
the axial cone, and we notice that clear diatomic perfora- 
tions, fractures, grooves, ete., are bright at the upper 
focus and dark at the lower focus; while the silex, ribs 
and thickenings of silex especially, are dark at the upper 
