owered until the thread cuts the perpendicular
lines drawn upon the sides of the camera. By this means a perfectly
horizontal plane is obtained, as true as with the best spirit-levels, and
in less time. By tying three knots in the silk at twelve inches distance
from the one bullet and from each other, we have a measure for stereoscopic
pictures; and by making the thread thirty-nine inches and two-tenths long
from one bullet to the centre of the other, we obtain a pendulum vibrating
seconds, which is useful in talking portraits; as it will continue
vibrating for ten minutes, if one bullet be merely hung over any point of
suspension."

Thus we obtain a levelling instrument, a chronometer, and a measure of
distances, at a cost considerably under one penny.

The above will more fully explain to your correspondent [Phi]. (Vol. vii.,
p. 505.) my reasons for the length of thread stated; and with respect to
the diagonal lines on the ground glass, it is not material what may be the
distance of the principal object, whether six feet or six hundred: for if
the cross lines, or any other lines drawn on the glass, cut the central
object in the picture at any particular part--for example, the window of
any particular house, or the branch of any tree,--then the camera may be
removed to higher or lower ground, several feet or inches, to the right or
to the left, and the same lines be made to cut the same objects, previously
noted; the elevation will then be the same, which completes all that is
required.

In most stereoscopic pictures, the distances are too wide. For a portrait,
two inches and half to three inches, at nine or twelve feet distant, is
enough; and for landscapes much less is required than is generally given,
for no very great accuracy is necessary. Three feet, at three hundred
yards, is quite enough; and four to six feet, at a mile, will do very well.
Let experiment determine: for every photographer must learn his profession
or amusement; there is no royal road to be depended on. But a small
aperture, a quarter of an inch diameter, may be considered a good practical
size for a lens of three and a quarter inches, depending on light and time:
the smaller the aperture, the longer the time; and no rules can be given by
any one who does not know the size and quality of the lenses employed.
Every one can make a few trials for himself, and find it out; which will be
more satisfactory than any instructions derived from books or