As I have
before said, the artist should be careful to see that the interior of
the camera is clean and free from dust, as the small particles flying
about, or set in motion by the sliding of the holder into the box,
attach themselves to the plate, and cause the little black spots, by
which an otherwise good picture is frequently spoiled. Care should
also be taken in withdrawing the dark slide, in front of the plate,
from the holder, as the same effect may be produced by a too hasty
movement. The lens is the last thing to be uncovered, by withdrawing
the cap c. fig. 5., which should not be done until you have placed the
sitter in the most desirable position. When, according to the judgment
and experience of the operator, the plate has remained long enough to
receive a good impression, the cap is replaced over the lens, and the
dark slide over the plate, which is then removed from the camera.

Daguerreotypists generally mark time by their watches, arriving at the
nearest possible period for producing a good picture by making several
trials. As a ready method of marking short intervals of time is,
however, a very important consideration, and as any instrument which
will enable an artist to arrive at the exact period, must be an
improvement, and worthy of universal adoption, I will here describe one
invented by Mr. Constable of England, which he calls a

Sand Clock, or Time Keeper.--"It consists of a glass tube, about twelve
inches long, by one in diameter, half filled with fine sand, similar to
that used for the ordinary minute glasses, and, like them, it has a
diaphram, with a small hole in the centre through which the sand runs.
The tube is attached to a board which revolves on a centre pin; on the
side is a graduated scale, divided into half seconds; the tube is also
provided with a moveable index. This instrument is attached, in a
conspicuous place, to the wall. The glass tube being revolved on its
centre, the index is set to the number of half seconds required, and
the sand running down, the required time is marked without the
possibility of error. In practice it will be found to be a far more
convenient instrument for the purpose than either a clock or a seconds
watch, and is applicable both for the camera and mercury box."

If the artist finds it desirable or necessary to take the object to be
copied in its right position, that is reverse the image on the
spectrum, he can do so by attaching a mirror (whi