80 
On the MicrosGojpe Prism 
[Monthly Microscopical 
Journal, Aug, 1, 1869. 
Fig. 1. 
the surface of the valves which led to any definite and exact know- 
ledge of the structure. In the short discussion on the paper, in 
which, by perhaps unavoidable circumstances, I was prevented from 
taking a part, it appeared that some microscopists had for years 
"considered the markings to be spherical." Here, no doubt, an 
erroneous method of illumination, though incapable of suppressing 
the whole of the truth, was yet insufficient to reveal " nothing but 
the truth," and hence Mr. Slack very naturally distrusts the usual 
mode of displaying diatom valves. 
By breaking up a valve of P. quadratum^ Mr. Wenham had 
obtained single spherules, and had also detached a line of spherules 
like a row of beads ; but this most conclusive evidence still fails to 
convince some observers that the law of Diatom structure is esta- 
blished. It is more than probable, how- 
ever, that their doubts will be set at 
rest by the single pencil of parallel 
light reflected from the equilateral 
prism. 
Since the paper was read, I have 
used the upper crown-glass hemisphere 
of the kettledrum as a Brewster's 
hemispherical prism, in which, says 
Brewster, the two convex surfaces are 
ground at the same time ; and Mr. Koss 
made a deep double concave flint lens, 
which is so placed within the converg- 
ing cone (Fig. 1) as to render the 
emergent rays both parallel and achromatic. In practice, how- 
ever, I find it easier to obtain parallel light by placing the lower 
hemisphere of the kettledrum, or any bull's-eye lens, between the 
lamp and the hemispherical prism (Fig. 2), and allowing the rays, 
Fig. 2. 
after crossing at the focus, to fall upon the plane surface of the 
prism. These diverging rays are just sufficiently shut up by the 
converging power of this prism as to be rendered parallel when 
