to regulate either 'depth of focus' or length of
exposure. The 'depth of focus' means the distance before and behind
the point in theoretically accurate focus, at which objects are
sufficiently focussed, for the purpose the photographer has in view.
This length is greatest when only the central portion of the lens is
in use. It is greatest with a pinhole, and least with a full
aperture. Hence a small stop is required if the picture is to include
near and far objects, while a large aperture may be used if all the
subject is far enough away to be in clear focus--say more than 25
feet--or if it is a flat surface. The small stop is also required when
the rising front or the swing back is in use. The power of regulating
the time of exposure is convenient for shortening long exposures in
dark interiors, or for lengthening inconveniently short exposures in
a bright light.

In practice it will be best to become familiar with the use of about
three stops, say the full aperture (perhaps F/5.6 or F/8), F/16, and
F/32.

For judging long exposures, the use of an actinometer (issued in many
inexpensive forms) is helpful.

A telephoto attachment increases the photographer's power of
rendering distant details on a large scale. The results are greatly
superior to enlargements of a small plate. It is, however, useless in
a wind, unless the camera is specially supported, and is otherwise
rather tricky to use. The traveller is strongly advised to master its
management at home. It should be adjusted by the maker to the camera
for which it is intended.

Unless a photographer's dark room can be had the developing of the
bulk is best left until the return home, but tests should be made to
see that the exposures are correct. A piece of ruby fabric or ruby
paper tied over an electric light will give a safe light after dark,
and 'Scalol' or some such one-solution developer which requires
merely the addition of water, will give all that is needed for
developing. For fixing use 4 oz. hypo to a pint of water.

In warm climates, use cold water. If it is not cool enough, the
gelatine of the negatives may give trouble. In that case, get colder
water, and use an alum bath. If water is precious, plates can be
sufficiently washed by moving them forward in succession, through
half-dozen soup plates filled with water.

If habitual use is not made of tabloid developers, &c., it is
advisable to have some in reserve, for use in the case of broken
bott