THE 
LONDON, EDINBURGH, ann DUBLIN 
PHILOSOPHICAL MAGAZINE 
AND 
JOURNAL OF SCIENCE. 
[FIFTH SERIES.] 
MAY 1885. 
XXXVI. Notes on the Use of Nicol’s Prism. By Jamus C. 
M‘CoynEL, B.A., Assistant Demonstrator at the Cavendish 
Laboratory, Cambridge*. 
1. On the Error in the Measurement of a Rotation of the Plane 
of Polarization caused by the Axis, about which the Nicol 
turns, not being parallel to the Incident Light. 
By eer O8E we have a beam of parallel light traversing a 
Nicol’s prism mounted in a graduated circle. Unless 
we have taken special precautions, we shall find that, when 
the Nicol is rotated, the plane of polarization of the emergent 
light turns through an angle somewhat different from that 
measured by the circle. For instance, if the axis of rotation 
of the Nicol is inclined 3° to the direction of the incident or 
emergent light, as large an error as 1° may be made in mea- 
suring a rotation of 60°. 
It is, however, tolerably well known that, as far as this 
error is proportional to the first power of the small angle of 
deviation, it may be eliminated by taking the mean of the 
readings in the two opposite positions of the Nicol circle 
separated from one another by nearly two right angles. It 
is of course only to a first approximation that this propor- 
tionality can be considered to hold good. And the main object 
of the present investigation is to determine, what is- the out- 
standing error we are liable to in assuming this proportionality ; 
or, in other words, with what accuracy we must adjust the 
Nicol circle, that this first approximation may be sufficient. 
* Communicated by the Physical Society. Read February 28, 1885. 
Phil. Mag. 8. 5. Vol. 19. No. 120. May 1885. Z 
