glass in the interior of the instrument. If the image is too large, we
push the camera back; if too small, push it up towards the picture and
focus again. The image is wrong side up, as we see; but if we take the
trouble to reverse the picture we are copying, it will appear in its
proper position in the camera. Having got an image of the right size, and
perfectly sharp, we will prepare a sensitive plate, which shall be placed
exactly where the ground glass now is, so that this same image shall be
printed on it.

For this purpose we must quit the warm precincts of the cheerful day, and
go into the narrow den where the deeds of darkness are done. Its
dimensions are of the smallest, and its aspect of the rudest. A feeble
yellow flame from a gas-light is all that illuminates it. All round us are
troughs and bottles and water-pipes, and ill-conditioned utensils of
various kinds. Everything is blackened with nitrate of silver; every form
of spot, of streak, of splash, of spatter, of stain, is to be seen upon
the floor, the walls, the shelves, the vessels. Leave all linen behind
you, ye who enter here, or at least protect it at every exposed point.
Cover your hands in gauntlets of India-rubber, if you would not utter Lady
Macbeth's soliloque over them when they come to the light of day. Defend
the nether garments with overalls, such as plain artisans are wont to
wear. Button the ancient coat over the candid shirt-front, and hold up the
retracted wristbands by elastic bands around the shirt-sleeve above the
elbow. Conscience and nitrate of silver are telltales that never forget
any tampering with them, and the broader the light the darker their
record. Now to our work.

Here is a square of crown glass three-fourths as large as a page of the
"Atlantic Monthly," if you happen to know that periodical. Let us brush it
carefully, that its surface may be free from dust. Now we take hold of it