PholtxjrnpUit' RvMt'arvhc* and Portrait ii re 287 



In the next section Gnlton i-pcords various methods he had hit upon for 

 superposing iniuges. Thus 



(a) Tie had used a sextant with its tolescoi)e attiwihetl. 



\h) Strips of mirror at various angles, tlitir sfvcrnl reflt'ctions W-itig 

 simultaneously viewed through a telescope. 



(c) A piece of glass inclined at a very acute angle to the liuo ot sjght 

 and a mirror heyond it also inclined hut in the opposite direction to the piece 

 of glass; the latter must he thin, a selected piece of the hest glass used for 

 covering microsco{)ic specimens. Several such pieces inclinea at different 

 angles may he used for multiple conijK)unding. 



{d) A divided lens like two stereoscopic Tenses brought close together in 

 front of the object-ghvas of a telescope (see our p. '285). 



(e) Glass negatives in separate magic lanterns all converging on the 

 same screen. 



{/) A camera with a long focus and many slightly divergent object- 

 glasses, each throwing an image of a separate glass negative upon a screen 

 (cf our p. 285). 



((/) A double image prism of Iceland spar. 



Of this Galton says (p. 138): 



"The liest instniinent I have a.s yet contrived and used fur optical superimposition is a 



'double-imago prism' of Iceland spar. The latest I have had has a clear aperture of a 



square half an inch in the side, and when held at ri^ht angles to the line of sight will separate 

 ordinary and extraordinary images to the amount of two inches, when the object viewed is 

 held at seventeen inches from the eye. This is quitt> sutiicient for working with carte-de- 

 visite portraits. One image is quite achromatic, the other shows a little colour. The diver- 

 gence 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 extraordinary image of the other, 

 and the composite may be viewe<l by the naked eye, or through a lens of long focus, or through 

 an openi-gla.ss (a telescope i.s not so good) fitte<l with a sufficiently long draw tulx* to see an 

 object at that short distance with distinctne.ss. Portraits 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 appearance of the composite. The 

 front and profile faces of two living persons sitting side by side, or one liehind the other, can 

 be easily superimposed by a double-image prism." 



The apparatus itself with accessories is figured and described in Galton's 

 paper, and remains after more than thirty years intact to this day in the 

 Galton Laboratory'. 



Galton remarks that the truth of the composite photograph can be assured 

 by the substantial agi-eement between the results from different batches of 

 components — " a perfect test of truth in all statistical conclusions." He tried 

 changing the order of exposure of the components and found substantial 

 identity' (p. 135). 



' There are in fact two such Iceland spar compounders. 



' It might seem that this point wanted greater experimental demonstration than the short 

 series Galton speaks of. I have, however, Galton's evidence before me; he took three portraits, 

 A, B, C, and compounded A + B and B + A, the composites are practically identical; then he 

 took A + B-^ C, A +C + B, B + A + C, B+C + A, C + A+B and C + B + A, and again the com- 

 posites are practically identical. He had thus good evidence that order of exposure did not 



