[Footnote 24: I have since had a more substantial instrument made with these and similar improvements.]
[Illustration: DIAGRAM SHOWING THE ESSENTIAL PARTS]
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 lantern; the whole can be moved to and fro.
_m_ The arm outside the camera attached to the axis of the reflector; by moving it, the reflector can be moved up or down.
_g_ A ground-glass screen on the roof, which receives the image when the reflector is turned down, as in the diagram.
_e_ The eye-hole through which the image is viewed on _g_; a thin piece of glass immediately below _e_, reflects the illuminated fiducial lines in the transparency at _f_, and gives them the appearance of lying upon _g_,--the distances _f e_ and _g e_ being made equal, the angle _f e g_ being made a right angle, and the plane of the thin piece of glass being made to bisect _f e g_.
_f_ Framework, adjustable, holding the transparency with the fiducial lines on it.
_t_ Framework, adjustable, holding the transparency of the portrait.
C is a travelling carriage that supports the portraits in turn, from which the composite has to be made. I work directly from the original negatives with transmitted light; but prints 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.