INVERCARGILL, NEW ZEALAND,
   November 6th, 1877.

  To CHARLES DARWIN, Esq.

SIR,—­Although a perfect stranger to you, and living on the reverse side of the globe, I have taken the liberty of writing to you on a small discovery I have made in binocular vision in the stereoscope.  I find by taking two ordinary carre-de-visite photos of two different persons’ faces, the portraits being about the same sizes, and looking about the same direction, and placing them in a stereoscope, the faces blend into one in a most remarkable manner, producing in the case of some ladies’ portraits, in every instance, a decided improvement in beauty.  The pictures were not taken in a binocular camera, and therefore do not stand out well, but by moving one or both until the eyes coincide in the stereoscope the pictures blend perfectly.  If taken in a binocular camera for the purpose, each person being taken on one half of the negative, I am sure the results would be still more striking.  Perhaps something might be made of this in regard to the expression of emotions in man and the lower animals, &c.  I have not time or opportunities to make experiments, but it seems to me something might be made of this by photographing the faces of different animals, different races of mankind, &c.  I think a stereoscopic view of one of the ape tribe and some low-caste human face would make a very curious mixture; also in the matter of crossing of animals and the resulting offspring.  It seems to me something also might result in photos of husband and wife and children, &c.  In any case, the results are curious, if it leads to nothing else.  Should this come to anything you will no doubt acknowledge myself as suggesting the experiment, and perhaps send me some of the results.  If not likely to come to anything, a reply would much oblige me.

 Yours very truly,
   A.L.  AUSTIN, C.E., F.R.A.S.

Dr. Carpenter informs me that the late Mr. Appold, the mechanician, used to combine two portraits of himself under the stereoscope.  The one had been taken with an assumed stern expression, the other with a smile, and this combination produced a curious and effective blending 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.