capable of adjustment of focus, and here again, strength and
simplicity should be looked for. It should be provided with effective
tripod legs, for studied exposures. Plates or flat films are
preferable to roll fills [2] which are difficult to manipulate away
from home. Flat films are less bulky and less breakable than glass,
and can be sent by post. They are supplied by the makers in packs of
12 for daylight loading into a film-pack adapter, which must be
provided to take the place of the ordinary dark slides for glass
plates. The lens should be a modern anastigmatic by a good maker. A
focal length of about six inches will be best for a quarter-plate
camera. A bad lens makes success impossible even by accident.

[2] Transcriber's note: 'fills' in the original text is possibly a
misprint for 'films'.

The stops will probably be of the Iris pattern, incorporated in the
lens and so not likely to be lost, as often happens with loose stops.

A few words on the theory and use of the stops and on the F-notation
may be of service. The speed of a photographic lens depends on the
ratio of the effective aperture to the focal length. Thus any two
lenses used at apertures of F/8, that is at apertures having
diameters one-eighth of their respective focal lengths, should be of
the same speed, though both lenses and apertures may be very
different. In a given lens, the speed varies directly with the area
of the aperture admitting the light, that is with the square of the
diameter of the aperture. The series of stops usually employed is
calculated so that each aperture is half the area of the preceding.
Stated in terms of the focal length they are known as F/5.6, F/8,
F/11.3, F/16, F/22.6, F/32, &c. Since the squares of those numbers,