should be done, and that the
screen should be replaced by the dark slide, and a brief exposure
given to the plate; then, that a fresh transparency should be
inserted, a fresh focusing adjustment made, and a second exposure
given, and so on. This, I say, is conceivable, but it would be very
inconvenient. The adjusting screws would be out of reach; the head
of the operator would be in an awkward position; and though these
two difficulties might be overcome in some degree, a serious risk of
an occasional shift of the plate during the frequent replacement of
the dark slide would remain. I avoid all this by making my
adjustments while the plate continues in position with its front open.
I do so through the help of a reflector temporarily interposed
between it and the lens. I do not use the ordinary focusing-screen
at all in making my adjustments, but one that is flush, or nearly so,
with the roof of the camera. When the reflector is interposed, the
image is wholly cut off from the sensitised plate, and is thrown
upwards against this focusing-screen, _g_. When the reflector is
withdrawn, the image falls on the plate. It is upon this
focusing-screen in the roof that I see the fiducial lines by which I
make all the adjustments. Nothing can be more convenient than the
position of this focusing-screen for working purposes. I look down
on the image as I do upon a book resting on a sloping desk, and all
the parts of the apparatus are within an easy arm's length.

My reflector in my present instrument is, I am a little ashamed to
confess, nothing better than a piece of looking-glass fixed to an
axle within the camera, near its top left-hand edge. One end of the
axle protrudes, and has a short arm; when I push the arm back, the
mirror is raised; when I push it forward it drops down. I used a
swing-glass because the swing action is very true, and as my
apparatus was merely a provisional working model made of soft wood,
I did not like to use sliding arrangements which might not have
acted truly, or I should certainly have employed a slide with a
rectangular glass prism, on account of the perfect reflection it
affords. And let me say, that a prism of 2 inches square in the side
is quite large enough for adjustment purposes, for it is only the
face of the portrait that is wanted to be seen. I chose my
looking-glass carefully, and selected a piece that was plane and
parallel. It has not too high a polish, and therefore does not give
tr