:j«m) 



Life ami Litterg of Francis (ialton 



cure the blurriiic. A print was then taken of the result, and this print 

 placed in the objective plane rotated through an angle 6^ in the opposite 

 direction to ^, is rephotographed. Then Galton discovered that with a certain 

 relation' lietween ^, and 6, the second photographing can l)e made to restore 

 linearity, or an original circle be converted into an ellipse. The accompanying 

 ditigrams indicate the three stages of Oalton's process. On the last diagram 

 he hatl written the words " Sliow this to Peai-son." He never did so and onlv 



Object 



Print of First Negative 

 Diagram iii. 



I'rint of Second Negative 

 Reduction of Circle to Ellipse 



after his death has his biographer discovered the large amount of time and 

 energy Galton spent over tliis matter; there are elaborate tables of 0, and 6 

 with the corresponding vertical and horizontal reductions. There is also tin 

 first draft of a paper intended for publication, but I cannot find that the 

 paper and the tables' were ever completed, still less published. Like so many 

 of Galton's ideiis it was simple and suggestive, but Galton was too old in 



' We have found that a single negative will suHice, if a second camera Ije euijiloyed t'> 

 photograph the image on the foea] plane screen of the first, this camera being fidjuste<l so that it'- 

 focal plane makes an angle 0, with the focal plane of the first camera. The difficulty lie.s in tin 

 length of exposure re<]uiKit« if the objectiveji of l>oth cameras are cut down so as to redui> 

 blurring to insignificance. Nothing is gained theoretically or practically by tilting as well ns 

 rotating the object plane of the first and the optiwil axis of the second camera. The theory is 

 an follows : I^et rf, and b, be respectively the disUnces from the optical centre of the first 

 camera t« the object plane and to the focal plane, and fi, and 6, the disUnces from the optical 

 centre of the second camera to the focal plane of the first camera and to iU own focal plane. 

 Then for rectilinearity in the photograph we must have 



d, tan 6, = 6, sin 6, ; 



and if R„ R^ be the vertical and horizontal scales of reduction, then 



X, = rf, dt/(b, 6,), /f» = R, cos *, cos tf,. 



Thus Jii, must lie le«s than R^, but this is no limitation as the object can always Imj turned 

 through a right angle. Actually the chief difiiculty lies in the suiUble choice of J, and fZ.,. The 

 api>aratus Ukes a simpler form if we keep the two optic axes in the same straight line, and th. 

 object perpendicular to them, but roUte the focal planes in opposite directions. 



' There is also a bundle entitled "Photographic lieduclion in breadth" with mcnlels in Iwth 

 wood and canJ of the propo8e<l camera apparatus. ,\s fjir as 1 can see Ualton always proposed 

 making an auxiliary intermediate print. 



