by the upper left-hand corner and pour some of this thin syrup-like fluid
upon it, inclining the plate gently from side to side, so that it may
spread evenly over the surface, and let the superfluous fluid drain back
from the right hand upper corner into the bottle. We keep the plate
rocking from side to side, so as to prevent the fluid running in lines, as
it has a tendency to do. The neglect of this precaution is evident in some
otherwise excellent photographs; we notice it, for instance, in Frith's
Abou Simbel, No. 1, the magnificent rock-temple façade. In less than a
minute the syrupy fluid has dried, and appears like 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