82 On the Microscope Prism ['j:'^^^Iiau^i,IIS^ 
meter, under a power of 12,000 linear, a set of 6 splierules at the 
top of 6 holloiv cones of memhrane, wliicli may be shown as brilliant 
objects on a dark ground, while at the same time they naturally 
prevent the direct light of the usual achromatic condenser from 
passing through them. If this is the true interpretation, and I 
believe it to be so, it is a curious fact that simple darkness in the 
hollow cones — the absence of light and not the presence of shadow 
— supplies our sldlled opticians with their best test in the shape of 
" a note of exclamation," having exquisite definition and apparent 
materiality. But if, instead of using direct light, we so place an 
equilateral prism as to throw a parallel beam of oblique light along 
the length of the scale, the shadow of the raised membrane which 
forms the hollow cone disappears, and we immediately get rid also 
of the interior darkness, and therefore of all trace of " exclamation " 
except that which almost naturally arises at the now novel sight of 
nothing but small spheres upon what we know to be a scale of the 
Podura ! The object seems to be — as by Mr. Wenham's method it 
really is — illuminated from above, and the " bright blue circular 
spots " of Mr. Wenham are seen by transmitted light and natural 
shadows to stand out in full relief as distinct spherical bodies. 
These spherules may often be distinctly seen on the margin of the 
scale, and in more than one instance I have seen them as detached 
bodies near the scale. 
Among the " variety of modes of illumination " alluded to by 
Mr. Beck, there are no doubt some which are calculated to mislead 
us ; but the equilateral prism is a safe guide, and much information 
respecting the structure of the Podura scale may be readily gathered 
by throwing the parallel beam of light in various directions on 
its surface. Most of the peculiar characteristics pointed out by 
Mr. Wenham become apparent, as well as the new features above 
described; and, notwithstanding the difference in our modes of 
examination, we come to the same conclusion that the markings are 
" not real spines," but " so incorporated with the membranes that 
separation cannot be effected." 
The equilateral prism which I used in the first instance for 
supplying a single pencil of parallel light is 5 inches long with 
one-inch faces. It was made about thirty years ago of the well- 
known white sand which abounds in my old parish of Stone. I now 
use with equal effect and easier management much smaller prisms 
of an inch and even half-an-inch in length, with inch and half-inch 
faces. Mr. Eoss has adapted these prisms to my microscope, 
mounted on a small arm with ball-and-socket joint. In a popular 
instrument the expensive luxuries of a mechanical sub-stage and 
elaborate condensers may be dispensed with, and the Prism Micro- 
scope, consisting of the body with its powers, a thin stage, and a 
two-inch equilateral prism, will look like a good working tool, and 
