Moura, Nove BO. of the Microscope. 261 
raised membrane disappears, and we immediately get rid also of 
the interior darkness, and therefore of all trace of ‘exclamation’ 
(markings), except that which almost naturally arises at the novel 
sight of nothing but small spheres upon what we know to be a scale 
of Podura..... 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.” * 
A very decided advance is here indicated. A pencil of rays, 
even though a rough approximation to real parallelism as obtained 
by a light placed in the focus of a simple plano-convex lens, is far 
more free from rays of confusion, so hostile to distinct definition, 
than ordinary condensed light. 
In 1862, Messrs. Powell and Lealand constructed for me an 
apparatus for emitting parallel rays, a description of which was 
forwarded last year to this Journal; and it was with parallel rays 
that the beads of a Podura scale were first seen in black dots in 
the place of the usual notes of admiration.} 
The advance of microscopical powers is still going forward. 
Mr. Wenham in his last communicationt has sketched the Podura 
markings something like an Ivishman’s “shillalah.” Our beautiful 
notes of admiration are now clubbed at the ends. Dr. Woodward’s 
photographs give also a distinct rounded head to many of them ; 
and Dr. Maddox’s sun-pictures scatter beads pretty thickly. It 
would seem the actinic glasses fail at present to perfect these views : 
illumination is a vexed question for the photographer, who has 
never yet succeeded in displaying the spherules of the Amphiplewra 
pellucida, which so many English observers have seen micro- 
scopically. Unless, therefore, the defining power of photography at 
least equals the human eye armed with the microscope, no reliable 
argument can be drawn from its failure or approximate revelations. 
Indeed the President notices that photography indicated hexagon 
forms as the correct appearance for the diatom beadings, which we 
now know is false. 
On the other hand, in 1854 Mr. Wenham obtained by photo- 
graphy, photographs of the Angulatum magnified 15,000 diameters, 
and stated that if ever the structure of these difficult tests is to be 
proved, it will be by the aid of photography.§ He further writes,— 
“ Dr. Pigott, by converting the microscope object-glass into a species 
of telescope and viewing distant and minute discs of light, professes 
by means of the ‘Aplanatic searcher’!!! (ste), to have discovered 
spherical error in all our best glasses, to the existence of which 
eyeryone else has hitherto been blind. Doubtless a very imposing 
demonstration may be made out of this. ... . ¥ 
If this is really the case, I must consider that Mr. Wenham 
has paid the highest compliment imaginable. However, he has 
* August, 1869. + 1862. t September. § July, 1870. 
