right-sided profile has to be combined with a
left-handed one, it must be pinned on one of the frames and viewed by
reflection from the mirror in the other. The apparatus I have drawn
is roughly made, and being chiefly of wood is rather clumsy, but it
acts well.]

Another instrument I have made consists of a piece of glass inclined
at a very acute angle to the line of sight, and of a mirror beyond it,
also inclined, but in the opposite direction to the line of sight.
Two rays of light will therefore reach the eye from each point of
the glass; the one has been reflected from its surface, and the
other has been first reflected from the mirror, and then transmitted
through the glass. The glass used should be extremely thin, to avoid
the blur due to double reflections; it may be a selected piece from
those made to cover microscopic specimens. The principle of the
instrument may be yet further developed by interposing additional
pieces of glass, successively less inclined to the line of sight,
and each reflecting a different portrait.

I have tried many other plans; indeed the possible methods of
optically superimposing two or more images are very numerous. Thus I
have used a sextant (with its telescope attached); also strips of
mirrors placed at different angles, their several reflections being
simultaneously viewed through a telescope. I have also used a
divided lens, like two stereoscopic lenses brought close together,
in front of the object glass of a telescope.

II. GENERIC IMAGES.

[_Extract from Proceedings Royal Institution, 25th April 1879_]

Our general impressions are founded upon blended memories, and these
latter will be the chief topic of the present discourse. An analogy
will be pointed out between these and the blended portraits first
described by myself a year ago under the name of "Composite Portraits,"
and specimens of the latter will be exhibited.

The physiological basis of memory is simple enough in its broad
outlines. Whenever any group of brain elements has been excited by a
sense impression, it becomes, so to speak, tender, and liable to be
easily thrown again into a similar state of excitement. If the new
cause of excitement differs from the original one, a memory is the
result. Whenever a single cause throws different groups of brain
elements simultaneously into excitement, the result must be a
blended memory.

We are familiar with the fact that faint memories are very apt to
become