PROCEEDINGS 
OF THE 
Cambridge Philosophical Society. 
_ Hexperiments with a prism of small angle. By G. F. C. SEARLE, 
Se.D., F.R.S., University Lecturer in Experimental Physics, Fellow 
of Peterhouse. 
[Read 23 February 1914.] 
§ 1. Introduction. The prisms of small angles supplied by 
opticians for use in spectacles are convenient in a number of 
optical experiments. When the refracting angle of a prism is so 
small that the circular measure, the tangent and the sine of the 
angle may be treated as identical to the accuracy which can be 
reached in the measurements, the mathematical calculations 
become so simple that the attention of the student is not diverted 
by them from the physical phenomena of refraction and deviation 
which he is studying. There is a close connexion between a 
prism of small angle and a simple lens of small aperture, for, 
when spherical aberration is neglected, the effect of the lens on 
rays proceeding from a point on its axis may be found by treating 
the part of the lens traversed by one of these rays as if it were 
part of a prism of small angle. Many lens problems are best 
solved by considering the deviation of a ray which meets a thin 
lens, or the principal planes of a thick lens, at a definite distance 
from the axis, and thus experiments such as those described below 
form a useful introduction to work with lenses. 
The apparatus is simple and can be freely handled by the 
student without risk of injury. Though the goniometer is some- 
what inferior in accuracy to a good spectrometer fitted with a pair 
of verniers, it has the great advantage that the readings can be 
very quickly taken, with the result that the student spends his 
time in studying optics rather than verniers. 
VOL. XVIII. PT. IV, al 
