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THE POPULAR SCIENCE MONTHLY. 



extraordinary image of the other, and the composite may 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 



Fig. 4. 



Fig. 5. 



Fig. 6. 



Fig. 4 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 iu 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 floor of the instrument is overlaid with cork covered with 

 black cloth, on which the components 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 composite purposes. Let A, B, C, . . . Y, Z, be the components. A is pinned 

 down, and B, C. . . . Y, Z, are successively 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 regis- 

 ter it. 



Fig. 5 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. 6, 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 , 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 aud 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.) 



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- 

 ance 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 



