ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
105 
logical specimens this work will bo very useful, as most of tho space is 
devoted to such preparations, the photographing of diatoms being only 
mentioned. 
At the end of the work is an appendix, showing how to prepare 
objects to be photographed. 
Podura Scale. — The following is the text of tho remarks made by 
Mr. T. F. Smith at the Society’s meeting last November (see this 
Journal, 1892, p. 908), which were illustrated by several photomicro- 
graphs : — 
In the papers read 16th December last and March 16th by the Hon. 
J. G. P. Yereker and Dr. A. Clifford Mercer on the subject of the structure 
of the Podura scale there is a direct conflict of evidence, in so much that 
while Mr. Yereker describes the structure as consisting of a hyaline 
beaded membrane having minute featherlets inserted in it, such featherlets 
being forked at the end, Dr. Mercer has seen nothing in the shape of 
spines or featherlets projecting from torn or folded scales, and doubts 
their existence. At the last meeting of this Society Mr. H. L. A. Wright 
throws his weight on the side of featherlets, but the evidence offered in 
support of this seems to have been of such an inconclusive character as 
to leave the question exactly as it stood before. It was my good (or 
bad) fortune some two or three years ago — when perhaps I was a little 
more sanguine of being able to solve the mystery of the scale than I 
am now — to devote a great deal of attention to this subject, and gained 
a little positive knowledge which I beg to offer to-night in hope the 
evidence produced may be able to carry the matter one step further. 
In estimating the value of the evidence already given us, it is 
necessary to consider the circumstances under which the images were 
produced ; and here I cannot help thinking that both observers have 
failed to take due advantage of the modern methods of illumination, 
or to get the best performance out of the objectives in their hands. 
Mr. Nelson’s remark when discussing Mr. Yereker’s paper, that the 
method of illumination used reduced the performance of an oil-immersion 
to rather less than a dry objective is so forcible and so true, that it 
would be an impertinence for me to add to it ; but from internal evi- 
dence offered by the reproduction of the prints referring to the Podura, 
I should say that Dr. Mercer has also been governed too much by the 
conventional appearance of the scale, and produced only the ordinary 
“exclamation marks” with the light streak on them, with which we 
have been so familiar for the last forty years. 
Now there are only two conditions under which you can produce 
these appearances with an oil-immersion. First, if the scale is on the 
cover you must throw the objective so much out of adjustment that 
the resulting image is valueless ; or, secondly, if the scale is on the slip, 
there is an air-space between it and the cover, and the lens performs, as 
Mr. Nelson says, rather worse than a dry objective. 
I submit two prints * here in support of my remarks — Nos. 1 and 2 : 
No. 1 taken with a dry 1/6-in., and showing the usual “markings,” 
and No. 2 taken with Zeiss’s 2-mm. apo. of 1*40 N.A., and in a fixed 
setting for a 160-mm. tube. In No. 2 you see the conventional “mark- 
ings ” have disappeared altogether, and in their place appears a series of 
* Copies of these prints are in the Society’s Library. 
