258 On the Advancing Aplanatic Power  ["sonmnal, Nov iO. 
to me with their new 1th the same black markings transformed into 
veritable translucent spines starting forth in a bold and beautiful 
solidity of aspect. In 1862, with their best 1th, newly made, of 
extraordinary aperture, the spherules of Formosum were magnifi- 
cently plain and decisive. 
An inductive argument in general for beaded structure of a 
transparent nature may be derived from the effects of rotating 
shadows. I beg to quote the following passage, which involves an 
important principle :— 
“The Podura beads sometimes exhibit black dots, crescentic 
shadows, and brilliant points of light. 
“Similar dots may be seen upon the beads of the Pleurosig- 
mata. Had we no visual direct proof of their sphericity, the sym- 
metrical shifting of the crescentic shadows according to the direction 
of the light would prove their shape.” * 
This announcement to the Microscopical Society in May, 1869, 
was followed up by confirmatory evidence in the July following. 
In a paper on the Podura, Mr. Wenham wrote :—*“ The scale 
appears marked over its surface with bright blue circular dots, 
which arise from the thick end of the note of admiration being the 
best to transmit light, showing that these are real prominences.” 
It is also stated, “ Let a strongly-marked and suitable Podura 
be now examined with the highest powers, say s'5 or 3/5, using the 
deepest eye-piece, and even lengthening the draw-tube (and many 
of our recent objectives will bear this admirably), the illumination 
being that of the achromatic condenser with adjustable apertures. 
.... Under this excessive amplitude” (about 11,000) “each 
individual marking retains its characteristic form, and though it is 
a body evidently having some bulk, not the most careful focussing 
can determine that it stands above the surface of the scale. The 
Binocular utterly fails in throwing up the spines in relief as from 
an underlying swrface..... We are unable to prove that they 
exist in the form of projections. .... If the markings were real 
spines they would stand out at the line of flexure like short bristles 
of a piece of hide folded together. But the markings ply round 
the sharp bend so closely that the keenest eye cannot detect any 
appreciable rib or projection.” 
As illustrative of the readings of what are considered indis- 
putably correct glasses, I shall venture to add a further extract from 
page 125, September, 1870, Journal, from the same writer. 
“Some years ago Mr. Ross showed me a Podwra scale mounted, 
uncovered, on a black disc, illuminated strictly as an opaque object 
by a Lieberkuhn, which he had adapted to a 3th of large aperture. 
The ‘note of admiration’ markings were finely displayed, and 
appeared like detached bodies due to surface elevations. This struc- 
* Received by the Society May 21, and published December 1, 1869. 
