May 23, 1878] 



NATURE 



99 



tions. 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 stereoscop)e 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. The latest that I have had were procured 

 for me by Mr. Tisley, optician, 172, 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 cartes-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 extra- 

 ordinary image of the other, and the composite may 



Fig. I. 



Fig. 



Fig. 3. 



Fig. I shows the simple apparatus which carries the prism and on which the 

 photograph is mounted. The former is set in a round box which can be 

 rotated in the ring at the end of the arm and can be clamped when 

 adjusted. The arm can be rotated and can also be pulled out or in if 

 desired, and clamped. The fljor of the instrument is overlaid with cork 

 covered with black cloth, on which the c )mponents can easily be fixed 

 by drawing pins. When using it one portrait is pinned down and the 

 other is moved near to it, overlapping its margin if necessary, until the eye 

 looking through the prism sees the required combination ; then the second 

 portrait is pinned down also. It may now receive its register-marks from 

 needles fixed in a hinged arm, and this is a more generally applicable 

 method than the plan with cross threads, already described, as any desired 

 feature— the nose, the ear, or the hand, may thus be selected for com- 

 posite purposes. Let a, b, c, . . . y, z, be the components, a is pinned 

 down, and B, c, . . v, z, are successively combined w.th a, and regis- 

 tered. 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 rod. Two of these carry a light frame covered with cork and 

 cloth, and the other carries Fig. 3, which is a frame having lenses of 

 different powers set into it, aad on which, or oa the thinl frame, a 

 small mirror inclined at 45* may be laid. When a portrait requires 

 foreshortening it can be pmned on one of these frames and be in- 

 clined to the line of sight ; when it is smaller than its fellow it can be 

 brought nearer to the eye and an appropriate lens interp;sed ; 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. 



be viewed with 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 further 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 and 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 45°. 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. 



Another instrument I hav« 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 oppo- 

 site direction to the line of sight. Two rays of light- 

 will therefore reach the eye from each point of the glass ; 

 Ihe one has been leflected from its surface, and the other 

 has been first reflected from the mirror, and then trans- 

 mitted through the glass. The glass used should be 

 extremely thin, to avoid the blur due to doubla 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 and their several reflections simultane- 

 ously 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. 



I Lave not yet had an opportunity of superimposing 

 Images by placing glass negatives in separate magic- 

 lanthoins, all converging upon the same screen ; but 

 this, or even a simple dioramic apparatus would be very 

 suitable for exhibiting composite effects to an audience, 

 and if the electric light were used for illumination the 

 effect on the screen could be photographed at once. It 

 would also be possible to construct a camera with a long 

 focus, and many slightly divergent object glasses, each 

 throwing an image of a separate glass negative upon the 

 same sensitised plate. 



The uses of composite portraits are many. They give 

 us typical pictures of different races of men, if derived 

 from a large number of individuals of those races taken 

 at random. An assurance of the truth of any of our 

 pictorial deductions is to be looked for in their substantial 

 agreement when differentbatches of components have been 

 dealt with, this being a perfect test of truth in asU statistical 

 conclusions. Again, we may select prevalent or strongly 

 marked types from among the men of the same race, just 

 as I have done with two of the types of cirixninals by 

 which this memoir is illustrated. 



Another use of this process is to obtain by photography 

 a really good likeness of a living person. The inferiority 

 of photographs to the best works of artists, so far as 

 resembJance is concerned, lies in their catching no more 

 than a single expression. If many photographs of a person 

 were taken at different times, perhaps even years apart, their 

 composite would possess that in which a single photograph 

 is deficient. I have already pointed out the experience of 

 Mr. Appold to this effect. The analytical tendency of the 

 mind is so strong that out of any tangle of superimposed 

 outlines it persists in dwelling preferably on some one of 

 them, singling it out and taking little heed of the rest. On 



