ON PEDIGREE STOCK RECORDS. 427 
way is to measure the heights of those two feet above gq and to roughly 
interpolate.| Measurements are also made between such marks on the 
wall as are visible, to furnish the scale of reduction at the distance of the 
wall from the camera. Fig. 2 represents a section of the installation on 
the same vertical scale as fig. 1, but the horizontal scale is much smaller 
and its internal proportions are not preserved, the primary object being 
to make acleardiagram. C is the object glass, D the point on the ground 
below it, g is the section of gq, here seen sideways, h is the projection of 
H upon the wall. Consequently C D=in reality 5 feet, DS=30 feet, 
S Q=5 feet, but the proportions are different in fig. 2 for the reasons just 
given. A line from C through Q determines the position of q, and gh 
being known by measurement, the position of i on the wall is known; 
then a line from C to h cuts the pathway at H, which gives the true 
position of the point where the vertical plane passing through C and p cuts 
the ‘hoof line’ on the pathway. Now M, the point on the pathway on 
which the vertical from P falls, lies in the same vertical plane as H, but a 
little further off from the camera, say 6 inches. This is a near enough 
Fig. 1. Photograph. Its Fig. 2. Section of installation on the 
scale is about 2 of that actu- same vertical scale as fig. 1. The horizontal 
ally used. scale is much smaller. 
estimate, as one or two inches of error here have no sensible influence on 
the result. So the position of H establishes that of M, and a line drawn 
from C through M determines that of m upon the wall as it would be seen 
in fig. 2, and consequently on the photograph as seen in fig. 1. mis not 
shown in the figure, as there is hardly room for it, and as it is not wanted 
in the simple way of working, which will immediately be explained. The 
height pm, as enlarged on the wall, has then to be reduced in the ratio of 
DM to DS, in order to obtain PM. The whole of this calculation is 
effected with the utmost ease by drawing the installation in its true pro- 
portions to a scale of ,!;th, using paper ruled into squares of ;'5th of an 
inch in the side, and converting the measurements made on the photographs 
into their corresponding values as projections upon the wall, reckoned in 
inches. The position of g is determined once for all on the paper by 
drawing a line from C through Q. A pin is inserted at C, and a loop 
made at one end of a thread is thrown over it. Q serves as the zero point 
both horizontally and vertically for all the working part of the diagram 
up to the line that represents the wall. But the zero point for this line 
is g. Then, the thread stretched through / determines H. Mis marked off 
at six divisions further on. The thread is now stretched through p, and 
the value of M P is read off at once. It is unnecessary here to enter more 
particularly into details, All other measurements in the plane of the 
