*joumawiiyTi^^^^ of Biotoms and Podura Scales. 27 
powers by the difficulty of obtaining light, but under the parabolic 
condenser, with a 3-th object-glass and a black field, this is a most 
brilliant and beautiful object. The so-termed spines still retain 
their peculiar form and decided materiahty ; and being as they are, 
either somewhat opaque or coloured, they retain the hght, and now 
appear luminous instead of dark, as before. In all other respects 
of interval, form, and position, they are the same as under trans- 
mitted Hght, and we are equally unable to prove that they exist in 
the form of projections. 
We will now consider the effects of fracture or dissection. Mount 
the scales from a recently-killed Podura on thin glass in the 
usual way. Take a fine needle, and roll and draw it amongst them, 
so as to mutilate them as much as possible, and place the thin glass 
on a slide for the examination. The first thing that strikes the 
observer, is the remarkable flaccidity of the scales. Comparing 
great things with small, they appear as limber as pieces of wetted 
paper, and roll up and crumple in a similar manner ; and at the 
spot where the specimens have been most bruised and broken, there 
is evidently moisture deposited on the glass, showing that the mem- 
branes contain fluid when the scale is attached to the living insect. 
The markings on the surface of such scales as are bruised, are not 
thrust sideways to any notable degree, they remain much in their 
original place, and appear merely to be flattened or mashed out to 
some extent, indicating a very firm attachment. Other scales that 
are doubled up, or folded over, show the markings exactly similar 
on both sides, and there is nothing peculiar at the line of flexure. 
If the markings were real spines, they would here stand out like 
the short bristles of a piece of hide when folded together. But the 
markings ply round the sharp bend so closely, that the keenest eye 
cannot detect any appreciable rib or projection at the edge of the fold. 
By transferring the investigation to a scale that has been 
ripped open, or torn across, nothing more can be learnt. The tear 
continues without interruption clean through the markings, one- 
half of which will be left on one piece, and the other half on the 
detached one, and no snags or projections can be seen on the clear 
edges of the suture. 
Not much therefore can be decided, as to the structure of this 
delicate object, by dissection, as in the case of the Diatoms, the 
brittle nature of whose built-up siliceous skeletons enables them to 
be broken up into the individual atoms of which they are composed. 
The only proof afforded by the experiment has been, that the mark- 
ings of the Podura are very strongly attached to the scales, and so 
incorporated with the membranes that separation cannot be effected. 
We must therefore again resort to illumination in hopes of solving 
the mystery. 
On March 26th, 1856, I read a paper before the Microscopical 
Society, " On a Method of Illuminating Opaque Objects under the 
