n the screen might be photographed then and there,
or that the same result may be much more easily obtained by a method
of successive photography, and I have exhibited many specimens made
on this principle. Photo-lithographs of some of these will be found
in the _Proceedings of the Royal Institution_, as illustrations of a
lecture I gave there "On Generic Images" in 1879.

The method I now use is much better than those previously described;
it leads to more accurate results, and is easier to manage. I will
exhibit and explain the apparatus as it stands, and will indicate
some improvements as I go on. The apparatus is here. I use it by
gaslight, and employ rapid dry plates, which, however, under the
conditions of a particularly small aperture and the character of the
light, require sixty seconds of total exposure. The apparatus is 4
feet long and 6-1/2 inches broad; it lies with its side along the
edge of the table at which I sit, and it is sloped towards me, so
that, by bending my neck slightly, I can bring my eye to an eye-hole,
where I watch the effect of the adjustments which my hands are free
to make. The entire management of the whole of these is within an
easy arm's length, and I complete the process without shifting my
seat.

The apparatus consists of three parts, A, B, and C. A is rigidly
fixed; it contains the dark slide and the contrivances by which the
position of the image can be viewed; the eye-hole, _e_, already
mentioned, being part of A. B is a travelling carriage that holds
the lens, and is connected by bellows-work with A. In my apparatus
it is pushed out and in, and clamped where desired, but it ought to
be moved altogether by pinion and rack-work.[24] The lens I use is a
I B Dallmeyer. Its focal length is appropriate to the size of the
instrument, and I find great convenience in a lens of wide aperture
when making the adjustments, as I then require plenty of light; but,
as to the photography, the smaller the aperture the better. The hole
in my stop is only two-tenths of an inch in diameter, and I believe
one-tenth would be more suitable.

[Footnote 24: I have since had a more substantial instrument made
with these and similar improvements.]

[Illustration: DIAGRAM SHOWING THE ESSENTIAL PARTS]

_Side View._

_End View._

A The body of the camera, which is fixed.

B Lens on a carriage, which can be
moved to and fro.

C Frame for the transparency, on a carriage
that also supports the l