Dr Searle, Experiments with a prism of small angle 169 
In the goniometers used at the Cavendish Laboratory, J is 
adjusted to be 40:00 cm. 
A mirror system may be constructed of two mirrors fixed to a 
block of wood. If glass silvered at the back is used, there will be 
a number of reflected images, which will not coincide unless the 
mirrors are absolutely “plane-parallel.” There will be no difficulty, 
however, in deciding if an image corresponds to two reflexions, 
one at each of the silvered surfaces, as this image is much brighter 
than the others. Multiple reflexions may be avoided by using two 
pieces of unsilvered plate glass, covered at the back with black 
varnish, or even, as a temporary measure, with vaseline. 
§12. Practical example. A mirror system consisting of two 
glass plates fixed to a block of wood was used. 
Distance of centre of pivot from goniometer scale =/= 40-00 cm. i 
The focal length of the goniometer lens was found by the method described 
in § 11. 
: . | Reading of ; Reading of | Change of goniometer 
Headline job goniometer Reading of .goniometer | reading for 1:5 cm. 
lass scale lass scale |* 
8 ; scale g scale on glass scale 
6°5 cm. 9°35 cm. 8:0 cm. 10-21 cm. | 0°86 cm. 
7:0 9°64 8°5 10°50 0°86 
1D 9:93 9:0 10°78 0°85 
| 
8:0 10°21 ) > ee 11:06 0°85 
Mean 0°855 cm. 
Hence, by (21), f=bl/2a=1°5 x 40/1°710=35-09 cm. 
The readings on the glass scale of the images due to the double mirror 
were 7°98 and 7°65 cm.; half the difference is 0°165cm. When the burner 
was moved away from the mirror system, as in Fig. 12, so as to illuminate 
the mirror AB only, the surviving image U was to the right of W, z.e. on the 
same side as the burner. The angle between the mirrors is therefore Jess 
than a right angle. Hence, by (20), if @ be the angle between the mirrors, 
p=dn —y/2f=4n —0:165/70-18= 42 — 0002351 
= 90° — 8’"1 =89° 51’-9, 
§ 13. Determination of the angle of a prism of nearly 90°. 
The method of §§ 10, 11 is easily adapted to the measurement of 
such a prism, provided the face opposite the reputed right angle is 
polished, as it must be if the prism is mtended to change the direc- 
tion of rays by refraction. Let ABC (Fig. 13) be a principal 
section of the prism, the angle A being nearly $7. Let PQRSXY 
be the path of a ray, the incidence at Q being nearly normal. By 
§ 10, if BAC = 177+ @, the angle between QR and XS is 26, and, if 
