l--in other words, we can say that for every film there
is a minimum amount of light action, and anything below this is of no use.
The exposure that enables the smallest amount of light action to take place
is termed the limit of the smallest useful exposure. {107}

There is also a maximum exposure in which the light affects practically all
the silver in the film, and any increased light action has no increased
effect. This is the limit of the greatest useful exposure.

In photo-telegraphy the duration of exposure, as already pointed out, is
determined by certain conditions connected with the transmitting apparatus,
and with conditions similar to those mentioned on page 75 the length of
exposure will vary roughly from 1-50th to 1-150th of a second.

The most suitable film to use for purposes of photo-telegraphy is one
having a fairly slow speed in which the range of exposure required comes
well within the limits of the film. There is no advantage in using a film
having a speed of, say, H. & D. 300 if good results can be obtained from
one with a speed of, say, H. & D. 200, as the use of the higher speed
increases the risk of overexposure. With the high-speeded films the
difficulties of development are also greatly increased, there being more
latitude in both exposure and development with the slower speeds, and
consequently a better chance of obtaining a good negative.

Another point, often puzzling to the beginner, and which increases the
difficulty of choosing a suitable make of film, is that, although one make
of film marked H. & D. 100 will give good results, another make, also
marked H. & D. 100, will give {108} very poor results. This is owing, not
to a poor quality film, as many suppose, but to the almost insurmountable
difficulty of makers being able to employ exactly the same standard of
light for testing purposes, so that although various makes may all be
standardised by the H. & D. method, films bearing the same speed numbers
may vary in their actual speed by as much as 30 to 50 per cent.

* * * * *

{109}

APPENDIX A

SELENIUM CELLS

Selenium is a non-metallic element, and was first discovered by Berzelius
in 1817, in the deposit from sulphuric acid chambers, which still continues
the source from which it is obtained for commercial purposes, although it
is found to a small extent in native sulphur. Its at. wt. is 79.2, and its
sp. gr. 4.8. Symbol, Se.

In its natural