I 



P/i<)to(/i'fipfn'c /icHcnrrhex and Port rait lire 'JltO 



pupil liiiy line and the line of the lips the same for all his components. But 

 the result of this vertical distance only heing the sjiine was a great diverHity 

 in hreadtlis, leading to an absence of sharpnesH in the outline ; thuH, hh 

 Galton expres8e<l it', the result wivs an aggreifute rather than a mean. He 

 now considered that the value of composites would l)e nuich increased, if 

 they were at the sjime time reduced to a common breadth as well as a 

 common vertical standard. Galton chase as his breadth the interpupillary 

 distance, but for some purposes it might be more useful to take, say, the 

 external ocular distance or even the breadth of fsice at ear level. The average 

 value of the selected vertiail and horizontal lengths was to be determined, 

 and each photograph reduced to these dimensions. Thus the problem becomes 

 precisely that referred to on our p. 45. A photographic armngement which 

 would act as a bi-projector was needful, and one must be devised in which 

 foreshortening would not be iiccompanied by any sensible blur. (Jalton first 

 considered that this result could be obtained by a form of pin-hole cjiinera 

 he had seen discussed in the Pltotographic News ; namely one in which the 

 pin-hole wius replaced by two adjustable diaphnigms. The first of these 

 diapluagms would contain a vertical slit, and have a motion horizontally but 

 perpendicular to the optical axis, and another motion along the optical axis ; 

 the second diaphragm would contain a horizontal slit, and have one motion 

 vertical and perpendicular to the optical axis, and another motion along the 

 optical axis. By proper adjustment of the two diaphragms, interchanging 

 them if needful, any desired modifications in height and breadth of the 

 object could be made. Theoretically the scheme is admirable ; it is 

 precisely that of the bi-projector referred to on p. 46, first ftn., except that 

 the beam of light is replaced in the latter by a " mechanical straight line." 

 The practical difficulty lies in the need of a very intense light on the object, 

 not only to reduce the long exposure, but to enable the operator to adjust 

 the image on the focal plane. When I discussed with Galton in 1903 the 

 possil)ility of double photographic reduction, he did not refer to this pin-hole 

 scheme, possibly he had di.scarded it aller trials 



Galton's notebooks and papers show that he spent in that year much time 

 over this problem of reducing photographically a circle to an arbitrary ellipse. 

 He proceeded, however, by an entirely difi'erent method. He proposed to 

 rotate his object plane round a vertical axis until it made an angle di with 

 the vertical plane, and then photograph it, trusting to stopping down to 



' Photographic S'ews, April 27, 1888. 



^ Trials have recently been made in the Galton Laboratory of this method of bi-projection&l 

 photography. To get rid of blurring the slits had to be extremely narrow, and thus a four 

 hours' exposure might be necessary for the reduction of a black and white drawing. It was 

 tlien found that the negative of the drawing had a series of light and dark bands across it. 

 I am not certain whether these are due to some diffraction effect, or to slight inequalities in 

 the breiwlth of the slits. I have found that a precisely similar system of bands, of course in 

 one direction oidy, may arise in photographing the sun with a focal plane shutter, when owing 

 to clearness of atmosphere it is neetlful to reduce the brea<lth of the slit in the shutter to a 

 minimum. Another objection to the method is that the circular dots used by draughtsmen for 

 points become ellipses, and vertical and horizontal lines do not remain of the same thickness, 

 but this objection applies to all methods of photographic bi-projection. 



38—2 



