58 
HINTS TO TRAVELLERS. 
Such tables are useful guides to the relative duration of exposure with 
diaphragms of different sizes applied to the same lens. They afford 
no clue, however, to time of exposure to be given with any particular 
lens or diaphragm. This can be best ascertained by experience, as 
duration of exposure of a plate or film in the camera depends on the 
sensitiveness of the plate, the time of day, the sun, the state of the 
atmosphere, the nearness or distance of the object to be photographed, 
etc. To take an extreme case of the difference of time required to impress; 
the plate with the image of an exterior view and that of an interior, a 
landscape open and well lighted may be taken in the fraction of a second, 
while a dimly-lighted interior with the same lens would require an hour, 
both being taken with plates coated with the same emulsion. The duration 
of exposure may be approximately estimated by using an exposure meter 
such as may be obtained from any photographic dealer’s. 
Sensitive plates or films .—Gelatine plates are now made commercially 
by a large number of firms and of great excellence; they keep indefinitely 
before exposure, and for a long time afterwards and before development, 
and under some circumstances (as for instantaneous pictures, portraits, 
and dimly-lighted interiors) will give results which could hardly be 
obtained at all on collodion. Gelatine plates are made of various degrees 
of sensitiveness; the slowest plates are best for ordinary landscape work. 
They are generally supplied in parcels of a dozen each, packed face to 
face with strips of folded paper between opposite edges. The card boxes 
in which they are usually packed are an insufficient protection against 
injury and damp. In all cases it is advisable, and for sea voyages and 
damp climates essential, to have each package of a dozen plates soldered 
down in a tin case, and afterwards packed in a light wooden box with 
tow or cotton wool, and the box screwed (not nailed) down. In packing* 
them up again after exposure or after development, a good plan (due to 
Captain Abney) is to provide oneself with a number of cardboard frames 
exactly the size of the plates, made of strips of card about l in. wide, one 
pf which is inserted between every two plates face to face. The packages 
thus made up should be soldered down again, and treated with at least 
as much care as the original plates. Should there be no available means 
of resoldering the boxes, it will be better to have tin boxes with the lid 
turned well down, the joinings to be closed by strong well-gummed 
paper. It will also be well to be provided with a supply of waterproof 
