PROCEEDINGS OF THE SOCIETY. 
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somewhat mixed up and distorted, but tho now slides show only the 
barest indications of tho structure underneath and tho upper is quite free 
from distortion. 
I said just now that even a water-immersion of about 1 * 15 N.A. failed 
to separate the two structures when at the normal distance on the valvo, 
not because the resolving power is insufficient, but because the depth of 
focus pierces through both layers ; which the next example will show you. 
This is the same as the last only taken with a dry 1/4 in., instead of the 
oil-immersion with which all the others are taken. Over the greater part 
of the valve there is no appearance of a double structure, but one strip 
is sufficiently isolated to allow it to be seen by itself, and I wish to point 
out that here the fibrils almost follow a straight line, while in the oil- 
immersion they are shown in short broken lengths, and on a line 
decidedly crooked. 
I take it that the 1/4 in. dry stands in about the same relation to 
P. formosum , with regard to resolving power, as the 1/1 2-in. oil immersion 
does to the finer forms of this genus, and analogy here would decide us 
to come to the conclusion that although the fibrils on the latter may 
appear straight they are not necessarily so. The next slide is a case in 
point, where we see straight fibrils projecting over a hole in tho valve, 
and was taken with the widest aperture of the oil-immersion. 
We now come to P. angulaium proper, and my justification in treating 
somewhat minutely on its structure will be found in the fact that from 
the time when it was first resolved at all, it has been taken as the test 
with which to measure every advance in the performance of objectives ; 
and before giving my own ideas of the structure, I propose to throw on 
the screen a few slides to represent the visual results obtained by 
objectives of different periods. 
The first is from a photograph taken by myself with an old 1/12 in. 
of Andrew Ross — date 1850 — and it was with a similar objective I 
suppose that Mr. Wenham took his celebrated photograph of this diatom. 
Unfortunately I have not done justice to this lens, but the characteristic 
appearance of P. angulatum at one particular focus is shown fairly enough. 
The next of the same diatom is also taken by myself with a Powell and 
Lealand 1/12-in. water-immersion of about 1-15 N.A. — date 1877. 
Next on the table there is a print taken by Dr. Woodward with one 
of the earlier oil-immersions of Zeiss, and the fourth a copy of the same 
diatom taken with an apochromatic oil-immersion of l* 30 N.A. by 
Dr. Roderick Zeiss. Now, practically there is no difference between the 
images taken with the first lens made in 1850 and the last in about 1888, 
and had we to judge alone by the appearances here presented, we should 
say there had been no advance in the definition of micro-objectives 
whatever — that is in the sense of showing something new and not in the 
relative sharpness of the photographic images ; but how unfair such an 
inference as this would be, I need not say to an assembly like this. We 
now come to a copy of one taken by Dr. Van Heurck with the new apo- 
chromatic of 1 * 63 N.A. ; but with all respect I must say that I am afraid 
he has not done justice either to himself or to the objective, but owing 
to a preconceived idea of the structure has focused down into a sort of 
no-structure-land where all the spectra meet and jostle each other. My 
warrant for saying this is his statement that he focused on the inter- 
