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Composite Portraiture
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and imperfect way of attaining the required result. It cannot of itself combine two images;
it can only place them so that the office of attempting to combine them may be undertaken
by the brain. Now the two separate impressions received by the brain through the
stereoscope do not seem to me to be relatively constant in their vividness, but sometimes
the image seen by the left eye prevails over that seen by the right, and vice versâ All the
other instruments I am about to describe accomplish that which the stereoscope fails to do;
they create true optical combinations. As regards other points in Mr. Austin’s letter, I
cannot think that the use of a binocular camera for taking the two portraits intended to be
combined into one by the stereoscope would be of importance. All that is wanted is that
the portraits should be nearly of the same size. In every other respect I cordially agree with
Mr. Austin.
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 450 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.
