124 Dr Searle, The determination of the focal 
directed. Thus, @ is the angle between the two positions of the 
arm in which (1) the image of F' and (2) the image of @ is focused | 
on the cross-wire. 
The goniometer devised by the author in conjunction with 
W. G. Pye and Co. is shown in Fig. 5*. 
The apparatus is arranged as in Fig. 6. The mirror system 
AWM is set up with its axis horizontal. A glass scale S is attached 
to a sliding carriage (Fig. 2), the direction of motion of the carriage 
being parallel to the axis of the mirror system. The divided face 
of the scale is turned towards the mirror and care is taken that 
the plane of this face is perpendicular to the axis of the system. 
The goniometer is then set with its axis coinciding with that of 
Fig. 6. 
the system and the scale S is adjusted by aid of the sliding carriage — 
so that the image of a nearly central dividing line is seen sharply 
focused (without parallax) upon the cross-wire of the goniometer. 
The end of the goniometer should be close to the scale S, care 
being taken that it is not so close that the arm comes into 
contact with the scale when it is turned about the pivot. The 
goniometer may, conveniently, be bolted to a table carried by 
a Compound Laboratory Stand (W. G. Pye and Co.). For accurate 
work the various pieces of apparatus must be firmly supported. 
The distance AF’ between the vertex A and the scale S, which 
has been set so that its divided face passes through Ff’, is measured 
by aid of an adjustable distance piece. Or the scale reading of the. 
carriage is taken and then the carriage is adjusted so that a rod 
of detinite length—10 or 20 cm.—touches A and F. From the 
two scale readings and the length of the rod, the distance AF is 
computed. 
The goniometer arm is then turned so that the cross-wire K 
is brought to coincidence with the images of a number of dividing 
* The instrument is described in my paper ‘‘Eixperiment on the harmonic 
motion of a rigid body” (Cambridge Phil. Soc. Proc. Vol. xvi. p. 31) and in the 
manual on Haperimental Harmonic Motion which it is hoped will be published 
in 1915. 
