Page 32 - The Wellington photographic handbook

obtained on ordinary plates, which cannot readily be secured with
isochromatic plates. Therefore, they use isochromatic plates
only when the subject seems specially to call for them. Others
use them on every possible occasion, and consider that they in-
variably give a better result, even without a screen. It is very
largely a question of personal taste, at least as far as the ordinary
run of photographic work is concerned ; but there are some cases,
as has been shown, in which there can be no possible doubt of the
advantages of the isochromatic plate.
Mention has just been made of the " screen " or " light filter,"
and this calls for some explanation. It has been pointed out that
ordinary plates are only very slightly sensitive to red, yellow and
green, but too much so to blue. This can be partly remedied, but
only partly, by the isochromatic process ; and in consequence,
WELLINGTON Iso-Speedy plates are much more sensitive to
yellow and green than, for example, WELLINGTON 'Xtra
Speedy plates. Neither are sensitive to red, as it is found that
in all ordinary photography little is gained by making a plate
red-sensitive, while it makes its manipulation much more difficult,
since it is then so easily fogged by the dark-room light. But
though the WELLINGTON Iso-Speedy plates are thus made
sensitive to yellow and green, they are still too sensitive to blue
and violet to give a truthful result, and so when it becomes impor-
tant to secure this, the photographer slips over or into his lens a
specially prepared yellow glass of film called a colour screen or
light filter. This yellow screen absorbs some of the blue and
violet rays, while not interfering with the yellow and green, and
its action, combined with the isochromatizing of the plate, enables
the result in every case, with the exception of the reds, to be an
accurate rendering.

The WELLINGTON Light Filter.
The WELLINGTON Light Filter has been carefully
adjusted for use with the WELLINGTON Iso-Speedy and
Ortho Process plates. It increases the exposure with those plates
to five times that which they need without the light filter.
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