■jSSiJuirMssa^T i^ot/aZ Microscoj^ical Society. 
II. — On the Diatom Prism, and the True Form of Diatom 
Markings, By the Eev. J. B. Keade, M.A., F.E.S., President 
of the Eoyal Microscopical Society. 
(Bead he/ore the Eoyal Microscopical Society, June 9, 1869.) 
The pages of our Transactions, from tlie commencement of our 
Society to the present time, bear ample evidence of the interest 
which is taken in the structure of the Diatom-valve, and of the 
Protean aspects which different observers have confidently recorded 
under different methods of illumination. In venturing to propose 
a new method of illumination and to describe new results, I must 
be permitted to copy the confidence of those who have preceded 
me, and to say that the usual methods of illumination are wrong in 
principle, and the consequent descriptions of the form of Diatom 
Markings are wrong in detail. But this, says one of my friends, is 
"a startler," and we all have to go to school again. I can only 
reply, that I have never left school, and the new lesson I have 
just learnt is not one of least interest, for it is admitted by those 
who have bestowed no unworthy labour on the minute structure of 
the diatom-valve, that the correct exposition of the structure 
involves a question quite as important, perhaps, as any we have to 
encounter in the whole course of vegetable physiology. It was 
only when I was imposed upon by lines, i. e. when I was taught to 
believe that on the valve of P. angulatum, for instancCj there are 
sets of three lines in the direction of the sides of an equilateral 
triangle, and formed by probably elevated ridges, that I proposed 
to obtain their shadows, not by a circle of light, as in the common 
" stop lens," but by three separate points of light of proper intensity 
in the kettledrum, to be placed by the revolution of the sub-stage 
at right-angles to the Hues to be resolved; and if this were the 
true structure, the principle of illumination is correct. The result 
also appeared to be satisfactory. The lines of shadows were readily 
made out, with the due arrangement of hexagonal markings, formed 
by the crossing of two equilateral triangles of these shadow-lines ; 
but, after all, so far as the eye was concerned, there was only an 
illustration of Berkeley's theory of " No matter," — shadow, without 
the substance. I have, however, at last seen the substance, and an 
exact knowledge of its form renders it absolutely necessary most 
materially to modify the mode of illumination. 
I will state at once — and I hope to prove to the Society, as I 
have proved to others, the truth of what I affirm — that the outer 
surfaces of the two valves of diatoms in the family NAVicuLEiE are 
covered with rows of siliceous hemispheres, inclined at varying 
angles both to each other and to the longitudinal division of the 
