Transactions of the 
[Monthly Microscopical 
Journal, July 1, 1869. 
sphere being Atts, and of the interval T%ths of an inch ; this, of 
course, makes the real diameter of the hemisphere TQ"wo"th of an 
inch, and therefore, with strigilis, among the largest in the range 
of valvular structure. The " stout costse " of Pinnularia major also 
follov?' the law of structure, and consist of very closely-packed 
spheres. When Mr. Sheppard, of Canterbury, saw these and other 
diatoms under the new illumination, he felt obliged to say that the 
microscope makes a new start on the Queen's birthday, 1869 ; and 
a young friend of mine, under fourteen years of age, exclaimed, when 
he saw the formosum, that " it looked like a plate of marbles." This, 
at all events, may be adduced as the evidence of an unprejudiced 
witness to the truthfulness of my story. Fleurosigma halticunij 
which we have all looked upon as presenting four-sided flattened 
pyramids, as described by Dr. WaUich, follows the same law of 
cell-formation as its congeners, the only modification being the crop- 
ping out of a rather larger portion of the sphere above the surface 
of the valve. 
It is amusing now to read of ingenious modes of playing with 
the illuminating rays, so that the eye, fortified by a little previous 
theory, may see at will, either elevations or depressions, triangular, 
quadrangular, or hexagonal dots, vdth rhomboids, pyramids, or 
spheres. But Truth is not so many-faced as this, and it is, there- 
fore, very satisfactory to find at the conclusion of Dr. Wallich's 
paper in the Transactions, that the editors have added an important 
note, which more than justifies my confidence in the accuracy of 
my descriptions. It is as follows : — " In the discussion which fol- 
lowed the reading of Dr. Wallich's paper, Mr. Wenham stated that, 
with an object-glass of his own construction, having a focal distance 
of about sVth of an inch and a large aperture, he had ascertained, 
beyond doubt, that in Fleurosigma angulatum, and some others, 
the valves are composed wholly of spherical particles of silex, 
possessing high refractive properties. And he showed how all the 
various optical appearances in the valves of the Diatomaceae might 
be reconciled with the supposition that their structure was uni- 
versally the same." Mr. Wenham will be glad to learn that, while 
the true valvular structure was revealed to him by the sVth of an 
inch, a power which few hands besides his own can make, and few 
observers can ever hope to possess, the diatom-prism, as an edu- 
cational adjunct, will enable all observers to see the exquisite 
structure of the coarser valves, for even a f rds of an inch by Wray, 
with the D and double D eye-piece, shows " the plate of marbles " 
on P. formosum with abundant light and perfect achromatism. 
It is needless to observe that deeper powers are required when 
attacking a valve hke N. rhomboides. Here the ^jth. is used, and 
I am sure that the exquisite beauty of this valve will be a treat to 
critical eyes. The acknowledged difficulty of resolving it arises 
