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Dr Searle, Experiments with a prism of small angle 167 
Thus PPFS=7— FPQR—FRQ=26. It is important to notice that 
the angle between PQ and RS does not depend upon the angle 
of incidence of the initial ray at Q. 
Ise, IL, 
In Fig. 11 the ray PQ falls first on the mirror AB. If, how- 
ever, a ray P'Q’ parallel to PQ falls first on AC, it will give rise 
to a ray RS’ which will be inclined to the ray P’Q’ at the angle 
20, but the deviation of R’S’ from P’Q’ will be in the opposite 
direction to the deviation of RS from PY. Hence a beam of rays 
parallel to PQ, which is wide enough to fall upon both mirrors, 
gives rise to two beams parallel to RS and RS’, these beams 
deviating by equal angles but in opposite directions from PQ. 
In this case we could find by how much the angle BAC differs 
from 90°, but we could not decide whether BAC is greater or less 
than 90°. To do this, we must ensure that the incident light falls 
on only one of the two mirrors—say on AB. 
Thus we can find the value of @ if we can measure the angle 
PFS between the initial and final rays. This can be done by the 
auto-colliimating goniometer. 
S11. Heaperimental details. The apparatus is arranged as 
indicated diagrammatically in Fig. 12. Here AB, AC are the 
two mirrors, LM is the lens of the goniometer, 7 is the totally 
reflecting prism of the auto-collimating device (see Fig. 2), and S 
is a source of light, preferably a small incandescent gas burner. 
When the burner is so close to 7’ that the rays which pass through 
T fill the lens LM with light, two images of the wire W will be 
seen, the wire being midway between the images. If, however, 
the burner is placed at some distance from 7’ in such a position 
that the rays transmitted by 7’ fall only on the mirror AB (as 
indicated in Fig. 12), only one image will be seen. If, as in Fig. 12, 
this single image U is to the left of W, then the angle BAC is 
greater than a right angle; if U is to the right of W, the angle 
is less than a right angle. I owe this suggestion to Mr S. D. 
Chalmers, When the burner is nearly in the proper position, the 
