320 Life ami Lettn-n of Francis Galtoii 



other animals" was published in Nahire on January 6, 1898'. It belongs 

 perhaps more closely to our chapters on biometry and heredity, but I have 

 mcluded it here xs concerned chiefly with ])hotograi)hic technujue. Oalton 

 jK)int^ out how fre«juently valuable horses and other show animals are photo- 

 graphed, but owing to the fact that there is no standardised method of 

 arranging animal and camera, it is not jiossible to take any meii.su rements on 

 these jihotographs. The st^mdardisation is a fairly easy matter, a rectangle 

 100 inches long and 20 inches broad is marked on the ground, by preference 

 in front of or parallel to a wall, upon which are two nails in the same 

 horizontal line at some distance apai't; a string terminating in two weights 

 is hung over these nails, and the vertical portions should he vertical on the 

 focal plane. A horse, say, is led on to the rectangle so that its feet all lie 

 within it, and so that the tips of its four hoofs and the short ends of the 

 rectangle are all visible in the focal plane. The optical centre of the camera 

 is 5 feet above the ground and 20 feet from the near side of the rectangle 

 measure<l on the ground; the optical axis is in a plane perj)endicular to the 

 long sides of the rectangle and this plane meets the rectangle in a line about 

 I foot from its central line and is parallel to the short sides. The camera is tilted 

 somewhat downwards, so tliat the rectangle and the hoi-se's hoofs shall be 

 visible in the photograph. The focal plane which must be adjustable is made 

 vertical by examining the vertical portions of the string which should be 

 vertical on the focal plane. 



If now the mid-point S on the line joining the tips of the fore-hoofs on 

 the photograjjh l)e joined to the mid-point 7' on the line joining the tips of 

 the hind-hoofs, this line ST provides the trace of the median plane of the 

 horse on the ground. Suppose first ST to be sensibly parallel to the long 

 sides of the ba.se rectangle and to meet the short sides in ^ and 7", then if 

 .S"7' be measuretl and equal s', it follows that all measurements on the photo- 

 graph in the median plane must be multiplied by 100/6"' to obtain actual 

 measurements on the horse. The photograph of coui-se only shows the section 

 on the median plane of a pencil of rays from the optic centre tangential to 

 the surface of the horse; but Galton calculates that in tlie case of a hoi-se 

 with the camera 20 feet away, this would not introduce an error of ^th of 

 an inch into the measurements. The point would be more serious in the case 

 of some fat stock with backs flat like tables, and in thisca.se a stud 2 inches 

 in height might be fixed in the median plane at any point of the back and 

 the measurement on the photograph taken to its top; afterwards 2 inches 

 would be deductefl from the deduced measurement. 



If ST be not pamllel to the long sides of the base rectangle, then its slope 

 to those sides can be found by producing it to meet them and dividing its 

 length in inches between them by 20 ; the secant of the angle corresponding to 

 this slope is the factor by which horizontal lines in the median plane must be 

 multiphetl in order to obtain their true value. Galton also indicates how 

 lines not parallel to the median plane can Ix^ obtained as from shoulder to 

 haunch bone (p. 232), but a discussion would carry us into too great detail. 



' Vol. Lvii, pp. 23(1-32. 



