156 Dr Searle, Experiments with a prism of small angle 
§ 2. The auto-collimating goniometer. The goniometer used 
in measuring the angles in the experiments to be described is shown 
in Fig. 1; it was designed in conjunction with W. G. Pye and Co, 
The base is formed of a strip of mahogany furnished at one end 
with a spherical pivot, and at the other with a cross-bar carrying 
a scale. Angles are measured by means of a moveable arm which | 
turns at dne end about the pivot, while the other end moves. 
over the scale on the cross-bar. The optical system consists of an | 
Fig. 1. 
achromatic lens, about 35 cm. in focal length, fixed to the arm” 
above the pivot and of a fine vertical wire held in an adjustable 
frame attached to the other end of the arm; the frame is adjusted 
so that the wire is in the focal plane of the lens. 
The spherical pivot is a phosphor-bronze ball attached to an 
adjustable fitting. The ball enters a conical hole turned out of a 
block of brass attached to the arm; the other end of the arm 
carries two brass feet which rest upon the cross-bar. 
The scale is provided with an anti-parallax mirror and is 
divided into millimetres and the ball is adjusted so that its centre 
is 40 cm. from the edge of the scale. The readings are taken by 
means. of a fine wire passing across an opening and stretched by 
a spring. As the scale can be read to ;1, cm., the angle can be 
read to zq459 radian or to about ->, degree. : 
The goniometer is converted into an auto-collimating instru- 
ment by the addition of the fitting shown in Fig. 2. The bar HF 
is clamped to the frame holding the vertical wire of the gonio- 
meter by the screws G, H. A vertical slit is cut in the upper 
edge of the bar and this slit is covered by a small totally reflecting 
prism 7’. When light from a source S falls upon the prism, it 
is internally reflected and passes out through the slit past the 
vertical wire WW. The beam of light then passes through the 
lens of the goniometer; if it suffers reflexion at a suitably placed 
