318 Life and Letters of F ramus (i alt on 



drawn by liaud itiid therefore inexactly, there wiis no inducement Ui conHider the poHsibilitie^ 

 of this converse piXK-ess, for which cxiictit luie in tlie |)ictiir(' is ('Hsciitiiil to hucccsh, but now 

 that photography has Iteoonu- common the old ditiiculty tins <lisup|H-ur<-(l, and tiie po.s.sibilitics 

 of the neglected pnK-ess well deserve consideration. The applications would be numerous and 

 especially valuable in deti^rmining and measuring restless nnimals in tlit-ir momentar}- attitudes, 

 or even when in rapid motion, which could not otherwise be mea»ure<l without difficulty, or 

 when in rapid motion he otherwiw measured at all. The obji'ct I have especially in view is to 

 establish a system of mensuring a large numlier of donu'stic animals of various })e<Iigree stocks, 

 whether horses, cattle, sheep, dogs, j)Oultry etc., in order to ju-ovido material to advance our 

 knowledge of heredity of a kind that is greatly neetUnl. It is not qualitative fact« and ex- 

 ceptional instances that are now wanted by students of liere<lity, but a large collection of 

 quantitative facts in the form of trustworthy measuri-nients. They are needed to determinu 

 with far greater precision than they are at pn^sent known the statistical laws and coefficients 

 of heredity. Among these are the conditions and rate of ' regre.ssion ' of the ortsj)ring of 

 exceptional parents; the gradual or sudden alterations of [xmition of the point towards which 

 regression tends, as the breed becomes more pure; the ivlative inQucnce of the male and female 

 parent in respect to various measurable peculiarities; the intensities of prepotencies; the fre- 

 quencies and magnitudes of sudden sports, and the degrees of tlieir suljsequent stability through 

 successive generations. ... I should add that the direct measurement of creatures so sensitive, 

 timid and sudden in their actions as thorough bre<l homes, who at the same time are often 

 vicious, is difficult and dangerous, similarly as regards bulls and some of the breeds of dogs. 

 Photography is a simpler, more exact and far safer method of measurement in these cases than 

 the ilirect application of rotl, tape and callipers." 



We shall consider later Galton's method of determining lengths parallel 

 to or nearly paiuUel to the focal plane of the camera. This he has published 

 (see onr p. 320). Ilis two-camera method of determining the three coordinates 

 in space of any |)oint of a subject has not, as far as I know, been publLilifd 

 and deserves a paragraph here. 



Diagram vii, figs. 1-5, is taken fnnn ( lalton's nianuscript. Fig. 3 represents 

 the plan on a working .scale ; M, , M, are the plans of the optical centres of the 

 two camera lenses; Ijak, fed are fiducial lines drawn on the base plane upon 

 which the object stands; b, a and c, d are fiducial points, M,a and ALc being 

 the traces of the vertical planes through the optical axes of the two cameras, 

 and these are so arranged that M,a and M...c are accurately at right angles 

 to bk and fd. Figs. 1 and 2 represent the two photographs, and p, and p., the 

 point P in them who-se coordinates are to be determined. From jj,, jy, per- 

 pendiculars p,<7 and pfave dropped on the images of the fiducial lines 6a and 

 cd in the photographs. But clearly (ig/ba = ng/ha. and f</cd = i'c/cd. Hence 

 g and f on the plan drawing can be scaled. Produce M,g and MS to meet in 

 Q, then Q is the plan of the given point P. If S be the perpendicular from 

 Q ou M,a, we may take SQ and aS for our coordinates x and y. Now draw 

 to the same scale an elevation (fig. 4) of the system on the vertical plan 

 through the optical axis; N,M, is tlie height of the optical centre, ST is the 

 elevation of P. Since Sa is known, by joining S to N, we obtain s. If the 

 elevation of P, or c, be TS, we require to determine ts, for knowing it we 

 have TS/t« = M,S/M,a. But ta m j>,(j the apparent height in the first photo- 

 graph altered in the ratio of ab to ab. Fig. 5 illustrates this clearly. Of course 

 \ -t settle the scale for the drawing-board ab by the value of the fiduciary 



' '• ab in the reticidation on the ba.se plane of the object photographed. 



Such is Galton's very simple process of tiiknig meivsurements on photographs 



