The best instrument I have as yet contrived and used for optical superimposition is a “double-image prism” of Iceland spar (see Fig., p. 228), formerly procured for me by the late Mr. Tisley, optician, Brompton Road.  They have a clear aperture of a square, half an inch in the side, and when held at right angles to the line of sight will separate the ordinary and extraordinary images to the amount of two inches, when the object viewed is held at seventeen inches from the eye.  This is quite sufficient for working with carte-de-visite portraits.  One image is quite achromatic, the other shows a little colour.  The divergence may be varied and adjusted by inclining the prism to the line of sight.  By its means the ordinary image of one component is thrown upon the extraordinary image of the other, and the composite may be viewed by the naked eye, or through a lens of long focus, or through an opera-glass (a telescope is not so good) fitted with a sufficiently long draw-tube to see an object at that short distance with distinctness.  Portraits of somewhat different sizes may be combined by placing the larger one farther from the eye, and a long face may be fitted to a short one by inclining and foreshortening the former.  The slight fault of focus thereby occasioned produces little or no sensible ill effect on the appearance 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,