ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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pensablo. Given a certain polarizing condition of the sky (i. e. blue, 
with more or less watery vapour — as either before or after a rain, snow, 
or fog), you can polarize very nicely with the analyser alone, and, if 
you want display of colour, put the selenite on top of the slide, or 
anywhere convenient to you — so it comes beneath the analyser. The 
colours (and crosses) will, of course, be somewhat fainter than when 
you use the polarizer too. In order to get the best display, it will 
be necessary to rotate both analyser and selenite until in the proper 
relative positions ; or, to speak more correctly, the relative position of 
the P.A. of the selenite to the beam of light from the mirror decides the 
more or less intense coloration. With any other sky, the polarization 
is not observed. This observation is useful in so far as to enable the 
possessors of Microscopes, without substage facilities, to polarize fairly 
well — under the circumstances — and the proper condition of the sky 
is often obtained in our latitude.” 
(4) Photomicrography. 
Photomicrography.* — Mr.T. Comber writes : — “ Photographing with 
the Microscope, or, as it is now the fashion to call it, “ Photomicro- 
graphy ,” has always had a great attraction to me. My first attempts at 
it were made so long ago as 1858, before the days of gelatino-bromide 
plates, and when the “ wet-collodion ” process was almost universal. 
At that time one of the difficulties to be contended with was tho 
want of coincidence of the actinic with the visual focus of the object- 
glass. Now most of our English makers can supply objectives specially 
corrected in this respect, so that when a visual image is focused on the 
ground glass, an image equally sharp in its actinic effects can be relied 
upon as thrown upon the sensitive plate. The apochromatics of Zeiss I 
have always found to be perfect in this respect. I should recommend 
any of you, who may be desirous of using your Microscope for photo- 
graphy, to be careful to obtain objectives corrected for the purpose ; but 
in case you may be tempted to use an objective that is not so corrected, 
I may mention the method by which, in those early days, we managed 
to overcome the difficulty ; the more so as the plan constitutes a good 
test to ascertain whether an objective said to be corrected for photo- 
graphy is in reality correctly corrected. Place a flat object on the stage, 
for choice a micrometer, and by putting a piece of card under one end 
of the slide, tilt it slightly up, so that the object no longer lies square to 
the axis of the Microscope, but is a little nearer on one side, a little 
further off on the other. Then focus carefully till the division of the 
micrometer scale lying in the centre of the field gives a sharp image on 
the ground glass, the other divisions will go gradually out of focus, those 
on one side being within, those on the other side beyond the focus. 
Next photograph the scale, and if any difference exists between the visual 
and actinic foci, it will be found that the centre division, which was sharp 
on the screen, is not sharp in the photograph, but that some other 
division more or less distant from the centre of the scale is. Eeplace 
the focusing screen and ascertain how much the fine-adjustment has to 
be moved to bring sharp on the screen the particular division that was 
* Journ. Liverpool Micr. Soc., i. (1891) pp. 99-110. 
