Photof/nipfii'r RfHcarrhrn mif/ Portra!liirr 



:!i:i 



with the iiitl of Mr T. 11. Diillint'yer for L'anyin<^ i>ut his |»n)jc'ct, luul this was 

 exhibited on the date just mentioned to the Koyal Photograjjhic Society; 

 Galton's paper is printed in Thr Photographic Jouimal, Vol. xxv, pp. 135- 

 38. The idea at the hottotn of Analytical Photof^raphy is extremely simple, 

 as most of (Jalton'.s metiu)d.s. A subject A and a subject li, taken in similar 

 positions and of similar size, have faint transparent positives and faint trans- 

 parent negatives taken of each. If now positive /I and negative yl l>e thrown 

 accurately adjusted on the same screen, they will antagonise each other and 

 give a iniiform grey backgi'ound. If further positive A and negative B be 

 thrown on the sjime screen, they will only antagonise one another where the 

 originals are identical ; where they are dirt'erent, they will only in part an- 

 tagonise each other. Thus the combination of positive A and negative B 

 gives a representation of their difference on a grey gi'ound. This Galton calls 

 the "transformer." If the transformer be thrown on the .screeVi with positive 

 li, it converts positive B into positive A. Similarly negative A and positive 

 B is the transformer, which superposed on positive A, converts it into B. 

 The two transformers are in fact jiositive and negative of the same difference. 

 In both cases the transformed portrait is that of a darkened subject. The 

 ct that our combination of faint positive and faint negative gives a uniform 

 eyor halftone is very important; because it follows that where our trans- 

 rmer adds nothing in the way of difference to A to make B, it will still 

 dd everywhere this grey or half tone. The transfomaed B will therefore be 

 I dnrh'ned picture of A. 



Galton illustrated this point by obtaining a 'real' scale of tints. He took 

 nine teetotums: the first had a white surface, the second a sector of 45° 

 painted black, the third two sectors of 45° black, the fourth three sectors and 



in 



m. 



Diagram v. Galton's photograph of a spinning wheel of tints. 



40 



