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PROCEEDINGS OF THE SOCIETY. 
experiments on the subject, the results of which I beg to lay before you 
to-night, and although the Podura scale may seem a trite subject, I 
have chosen it, first because its appearance is known to all microscopists, 
and secondly, because the slighest divergence of focus is instantly shown 
on the negative and the resulting prints. 
Prints Nos. 1 and 1a to 4 and 4a are taken with four lenses of 1/6 in. 
focus, two of them under, and two over-corrected, as far as the scale is 
concerned, and in embracing the two extremes it may be fairly argued 
that any intermediate correction would produce the same results. 
Further, to make the selection more comprehensive, one of each correc- 
tion is an old lens, while the other two are also of opposite corrections, 
but made partly of the new Jena glass. 
Now the quality of the image varies with the quality of the objective, 
but in all cases the result is the same. No screen or light-filter has 
been used, but when the negative is taken on an isochromatic plate the 
image in sharpness is an exact counterpart of that on the camera screen, 
with no falling away at the edges as described by Dr. Piffard ; but 
when, on the other hand, the negative is taken with the same focus 
and lighting on an ordinary Ilford plate the image comes out all fluff as 
shown on the right-hand print on each card. 
After experimenting on the four dry lenses, I went on to photograph 
the same scale with a cheap oil-immerson objective of 1/12 in. focus, 
using one colour-correct and one ordinary plate as before, but still 
without any screen, and here, to my surprise, both negatives came out in 
the same sharp focus. Thinking this might be due to the low magnifica- 
tion used — only 1150 diameters, the same as the others — I took the 
same scale at 2300 diameters on an Ilford ordinary plate and found that 
also came out perfectly true to focus. This, I think, is an interesting 
discovery, and proves that it is possible to make an ordinary achromatic 
oil-immersion lens of pure correction, which without fluorite shall 
have the visual and actinic foci identical, no matter what the magnifica- 
tion or what the plate used, whether colour-correct or otherwise. I 
have tried many oil-immersion lenses — not apochromatic — photographi- 
cally on ordinary plates, and this is the first I have found correct with 
high magnification, and I think it only fair to say that the makers of 
the lens in question are Messrs. Swift and Son. The question may be 
asked here, why it is not possible to make dry achromatic lenses of great 
aperture equally true photographically ; but I have been told that there 
are certain difficulties owing to the greater divergence of the rays from 
a dry objective. This last question, however, does not count if isochro- 
matic plates are used, and these are now so cheap that there is no 
motive in using any other. 
To sum up then, my conclusion from the evidence I have been able 
to collect is this : — 
It is possible to construct an oil-immersion objective without 
fluorite, the actinic focus of which shall be identical with the visual, 
without the use of any special plate, screen or ray-filter ; but that any 
oil-immersion lens not so corrected will yet produce a sharp image 
photographically if isochromatic plates only are used. 
Any ordinary achromatic dry lens, whether under or over-corrected, 
will also produce a sharp photographic image on isochromatic plates — 
