ng of the two.

Convenient as the stereoscope is, owing to its accessibility, for
determining whether any two portraits are suitable in size and
attitude to form a good composite, it is nevertheless a makeshift
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 versa_. 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
appear