ike a film of transparent
varnish on the glass plate. We now place it on a flat double hook of gutta
percha and lower it gently into the nitrate-of-silver bath. As it must
remain there three or four minutes, we will pass away the time in
explaining what has been already done.

The syrupy fluid was _iodized collodion_. This is made by dissolving
gun-cotton in ether with alcohol, and adding some iodide of ammonium. When
a thin layer of this fluid is poured on the glass plate, the ether and
alcohol evaporate very speedily, and leave a closely adherent film of
organic matter derived from the cotton, and containing the iodide of
ammonium. We have plunged this into the bath, which contains chiefly
nitrate of silver, but also some iodide of silver,--knowing that a
decomposition will take place, in consequence of which the iodide of
ammonium will become changed to the iodide of silver, which will now fill
the pores of the collodion film. The iodide of silver is eminently
sensitive to light. The use of the collodion is to furnish a delicate,
homogeneous, adhesive, colorless layer in which the iodide may be
deposited. Its organic nature may favor the action of light upon the
iodide of silver.

While we have been talking and waiting, the process just described has
been going on, and we are now ready to take the glass plate out of the
nitrate-of-silver bath. It is wholly changed in aspect. The film has
become in appearance like a boiled white of egg, so that the glass
produces rather the effect of porcelain, as we look at it. Open no door
now! Let in no glimpse of day, or the charm is broken in an instant! No
Sultana was ever veiled from the light of heaven as this milky tablet we
hold must be. But we must carry it to the camera which stands waiting for
it in the blaze of high noon. To do this we first carefully place it in
this narrow case, called a _shield_, where it lies safe in utter darkness.
We now carry it to the camera, and, having removed the ground glass on
which the camera-picture had been brought to an exact focus, we drop the
shield containing the sensitive plate into the groove the glass occupied.
Then we pull out a slide, as the blanket is taken from a horse before he
starts. There is nothing now but to remove the brass cap from the lens.
That is giving the word Go! It is a tremulous moment for the beginner.

As we lift the brass cap, we begin to count seconds,--by a watch, if we
are naturally unrhythmical,--by the