ance of the composite.

The front, or the profile, faces of two living persons sitting side
by side or one behind the other, can be easily superimposed by a
double-image prism. Two such prisms set one behind the other can be
made to give four images of equal brightness, occupying the four
corners of a rhombus whose acute angles are 45 deg. Three prisms will
give eight images, but this is practically not a good combination;
the images fail in distinctness, and are too near together for use.
Again, each lens of a stereoscope of long focus can have one or a
pair of these prisms attached to it, and four or eight images may be
thus combined.

[Illustration: Fig. 1 shows the simple apparatus which carries the
prism and on which the photograph is mounted. The former is set in a
round box which can be rotated in the ring at the end of the arm and
can be clamped when adjusted. The arm can be rotated and can also be
pulled out or in if desired, and clamped. The floor of the
instrument is overlaid with cork covered with black cloth, on which
the components can easily be fixed by drawing-pins. When using it,
one portrait is pinned down and the other is moved near to it,
overlapping its margin if necessary, until the eye looking through
the prism sees the required combination; then the second portrait is
pinned down also. It may now receive its register-marks from needles
fixed in a hinged arm, and this is a more generally applicable
method than the plan with cross threads, already described, as any
desired feature--the nose, the ear, or the hand, may thus be
selected for composite purposes. Let A, B, C, ... Y, Z, be the
components. A is pinned down, and B, C, ... Y, Z, are successfully
combined with A, and registered. Then before removing Z, take away A
and substitute any other of the already registered portraits, say B,
by combining it with Z; lastly, remove Z and substitute A by
combining it with B, and register it. Fig. 2 shows one of three
similarly jointed arms, which clamp on to the vertical covered with
cork and cloth, and the other carries Fig. 3, which is a frame
having lenses of different powers set into it, and on which, or on
the third frame, a small mirror inclined at 45 deg. may be laid. When a
portrait requires foreshortening it can be pinned on one of these
frames and be inclined to the line of sight; when it is smaller than
its fellow it can be brought nearer to the eye and an appropriate
lens interposed; when a