142 
PROCEEDINGS OF THE SOCIETY. 
were mostly confined to the structure of P. formosum as the coarser form 
and one likely by analogy to determine that of the finer forms of the 
same genus. At that time I had no positive evidence to offer as to the 
structure of P. angulatum, and so far my case was incomplete ; but since 
then further investigation has given me the evidence required to complete 
my case, and this I beg to lay before you to night. 
In the paper read I attempted to prove that each valve of this genus 
was composed of at least two layers of structure, differing from each 
other, that each side of the valve had different curves and each curve its 
distinctive structure. Of these layers the inner one was the more robust 
of the two, and the outer the one from which all my examples of torn 
structure had been taken. I further said that this outer layer consisted 
of bars of silex set lengthways on the valve in such a manner that the 
alternate interspaces allowed an image to be formed between them, giving 
rise to the well-known diagonal ‘ markings,’ an example of which I 
showed that night under the Microscope. Subsequently to this I sent 
a paper to the New York Microscopical Society* on the same subject ; 
in which I claimed that my results were the outcome of a large illumi- 
nating cone of central light, but in a paper by Dr. Cox,f read on the 
same night, strong doubts were raised as to my light being either central 
or consisting of a large cone. 
The greater part of Dr. Cox’s paper has been reproduced verbatim — 
without any quotation from my own paper to accompany it — in the 
Journal J of this Society, and I think it is only fair to myself that I 
should be allowed to make some remarks to show that I was not so 
much mistaken, as to the character of my illumination, as Dr. Cox seems 
to think. He says, ‘ I am bound in candour to say that in most of 
Mr. Smith’s prints I recognize similar effects to those which in my 
own work I attribute to oblique light.’ And again : 4 Going back from 
these to prints Nos. 10 and 11, we now find reason to accept these also 
as evidence of the same structure, though distorted by obliquity of light, 
so that they would not have been satisfactory taken by themselves.’ 
Let me admit at once, that judging the two prints by their appearance 
only, and not taking into consideration the circumstances under which 
they were produced, Dr. Cox is quite justified in his strictures ; but let 
it be remembered also that w r e are here dealing with a valve in which 
the layers lie so close together, that even a water-immersion objective 
of about 1*15 N. A. will not separate them when at the normal distance 
from each other. Then again there was the necessity of getting contrast 
enough to produce an image of sufficient density to print from, and at 
that time my photographic skill was not equal to getting this without 
stopping down somewhat the aperture of the sub-stage conden er. But 
while admitting the reasonableness of the conclusions, I was certain of 
the character of my illumination, and it only remained for me to try to 
produce photographically the same results as I was able to do visually. 
In this I have succeeded, and will now place the same objects, taken with 
the full aperture of the condenser, side by side of the old ones on the 
screen. Nos. 10 and 11 of my paper were taken, as I said, with the 
aperture of the condenser stopped down, and certainly the structure is 
* Journ. New York Micr. Soc., vii. (1891) p. Gl. f Tom. cit., p. 73. 
X See this Journal, 1891, p. 657. 
