film, and the characteristics of the film itself, are all factors
which have a decided bearing upon the quality of the results obtained, and
the film found to be most suitable in one case will perhaps give very
unsatisfactory results in another.

In photo-telegraphy the length of exposure is determined by the time taken
by the transmitting stylus to trace over a conducting strip on the metal
print, and this time, of course, varies with the density of the image and
also with the speed of transmission.

The film in ordinary photography is chosen with regard to the subject and
the existing light conditions, and the amount of light admitted to the film
and the length of exposure are regulated accordingly. No such latitude is,
however, possible in photo-telegraphy. With each set of apparatus {104} the
various factors, such as the light value, the amount of light admitted to
the film, and the length of exposure, will be practically fixed quantities,
and the film that will give the most satisfactory results under these fixed
conditions can only be found by the rough-and-ready method of "trial and
error."

The films in common use are manufactured in four qualities, namely,
ordinary, studio, rapid, and extra rapid. These terms should really relate
to the light sensitiveness of the film (or, as it is technically termed,
the speed), but at the best they are a rough and very unsatisfactory guide,
for the reason that some unscrupulous makers, purely for business purposes,
do not hesitate to label their films and plates as slow, rapid, etc.,
without troubling to make any tests for correct classification.

The speed of photographic films and plates is generally indicated by a
number, and the system of standardisation adopted by the majority of makers
in this country is that originated by Messrs. Hurter & Driffield,
abbreviated H. & D. In their system the speed of the film and the exposure
varies in geometrical proportion, a film marked H. & D. 50 requiring double
the exposure of one marked H. & D. 100. The highest number always denotes
the highest speed, and the exposure varies inversely with the speed.

Besides the Hurter & Driffield method of {105} obtaining the speed numbers
of plates and films adopted by a large number of makers in this country,
there are also two standard English systems known as the W.P. No. (Watkin's
power number) and Wynne F. No., both of which are used to a fair extent.

The "Actinograph" number or speed