ve the
purpose of the key K, and could so arrange matters that a minute portion of
the photograph could be transmitted separately but in succession, and that
each portion of the photograph having the same density could be given the
same signal, then it would only be necessary to have apparatus at the
receiving station capable of arranging the signals in proper sequence (each
signal recorded being the same size and having the same density as the
transmitted portion of the photograph) in order to receive a facsimile of
the picture transmitted.

The following method of preparing the photograph[3] is one that has been
adopted in several {15} systems of photo-telegraphy, and is the only one at
all suitable for wireless transmission. The photograph or picture which is
to be transmitted is fastened out perfectly flat upon a copying-board. A
strong light is placed on either side of this copying board, and is
concentrated upon the picture by means of reflectors. The camera which is
used for copying has a single line screen interposed between the lens and
sensitised plate, and the effect of this screen is to break the picture up
into parallel lines. Thus a white portion of the photograph would consist
of very narrow lines wide apart, while the dark portion would be made up of
wide lines close together; a black part would appear solid and show no
lines at all. From this line negative it will be necessary to take off a
print upon a specially prepared sheet of metal. This consists of a sheet of
thick lead- or tinfoil, coated upon one side with a thin film of glue to
which bichromate of potash has been added; the bichromate possessing the
property of rendering the glue waterproof when acted upon by light. The
print can be taken off by artificial light (arc lamps being generally
used), but the exact time to allow for printing can only be found by
experiment, as it varies considerably according to the thickness of the
film. The printing finished, the metal print is washed under running water,
when all those parts not acted upon by light, _i.e._ the parts between the
lines, are {16} washed away, leaving the bare metal. We have now an image
composed of numerous bands of insulating material (each band varying in
width according to the density of the photograph at any point from which it
is prepared) attached to a metal base, so that each band of insulating
material is separated by a band of conducting material. It is, of course,
obvious th