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Appendix
can be used with light falling on their face. For convenience of description I will confine
myself to the first instance only, and will therefore speak of C as the carriage that supports
the frame that holds the negative transparencies. C can be pushed along the board and be
clamped anywhere, and it has a rack and pinion adjustment; but it should have been made
movable by rack and pinion along the whole length of the board. The frame for the
transparencies has the same movements of adjustment as those in the stage of a
microscope. It rotates round a hollow axis, through which a beam of light is thrown, and
independent movements in the plane, at right angles to the axis, can be given to it in two
directions, at right angles to one another, by turning two separate screws. The beam of
light is furnished by three gas-burners, and it passes through a condenser. The gas is
supplied through a flexible tube that does not interfere with the movements of C, and it is
governed by a stop-cock in front of the operator.
The apparatus, so far as it has been described with any detail, and ignoring what was
said about an eye-hole, is little else than a modified copying-camera, by which an image of
the transparency could be thrown on the ordinary focusing-screen, and be altered in scale
and position until it was adjusted to fiducial lines drawn on the screen. It is conceivable
that this 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.
