398 
Transactions of the Society. 
from the edges, a common and uniform appearance is presented. Both 
this and the fracture markings are always to be seen in specimens 
such as are here shown. 
Although this state of the Podura scale appears to have escaped 
notice, all the slides I have seen possess many examples confirming 
this view. The cleavage of the denser membrane invariably presents 
the same structural appearance. This separation of the two mem- 
branes is common with the smaller scale, and, so far as my experience 
goes, entirely absent in the larger ones. 
The distinction of colour between the two membranes is more 
perceptible with apochromatic objectives ; nevertheless, it can be seen 
with achromatics and daylight illumination. 
Probably no object of any importance concerning insect anatomy 
is served by this explanation ; I cannot help thinking, however, some 
light is thrown on the interpretation of appearances of objects when 
viewed under the Microscope ; seeing a minute perforation when focused 
on the upper plane presents, not a clear transmission of light, as 
might naturally be expected, but an opaque or dark stroke. 
Those who are inclined to regard these appearances as having 
been produced by the presence of some foreign fluid will do well to 
consider the following points : — 
The cleavages are all of the same character. 
The edges of the scales when the upper membrane is removed 
present a uniform appearance, one that could never be invariably 
produced by a fluid flowing over the scale. In the perfect scale the 
markings are perpendicular, whereas in the imperfect ones at these 
edges they are segments of a circle diagonally arranged. 
If there is a substance flowing over and obliterating the markings, 
why is it that the markings of a scale in optical contact with it, but 
below the scale, are clearly visible through it, without the faintest 
indication of interference lines, thus showing the delicate membrane 
to be perfectly transparent ? 
Many portions of the upper membrane become ruptured and can 
be seen scattered over the slide and adhering to the cover-glass. 
When these fragments are present, even if such scales are not in the 
field of observation, their existence on the slide can be predicted. 
Sometimes there are faint markings which, examined hastily, might 
be mistaken for obliterated exclamation marks ; these are due to the 
greater and lesser density of the lower membrane caused by the removal 
of the upper one. When photographed the illusion vanishes. 
On the scale photographed there is no indication of any foreign 
fluid present likely to produce these appearances. If such a fluid 
existed in small quantities it should show a clearly defined margin. 
This is also absent. 
If this invisible fluid is present, producing such extraordinary 
uniform results, it must be by the operation of some unfamiliar 
attributes. 
