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 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.