colours which accompany them in calcareous spar. 291 
I shall now conclude this letter with the description of ano- 
ther instrument which I have found of great advantage in 
carrying on very delicate experiments on the polarisation of 
light. In comparing the quantities of light polarised in the 
plane of reflection by different metals, we derive very little 
aid from examining the partial evanescence of one of the 
images. The intensity of the complementary colours is a 
much more delicate measure of the portion that has received 
this character. The method of doing this is shown in Fig. 12, 
where ABCD is a tube about eight or nine inches long sup- 
ported upon a stand. An equal and unbroken plate of sul- 
phate of lime which gives an uniform tint in every part of its 
surface, is cemented with Canada balsam between two plates 
of parallel glass, and is placed at the end AB of the tube 
exposing two circular apertures m, n, to the incident rays. 
At the other end of the tube is a piece of black glass op , in- 
clined at an angle of 33 0 to the axis of the tube, and having 
a motion of rotation round that axis. When polarised light 
is transmitted through the apertures m, n, and reflected from 
the surface op to the eye, these apertures will appear equally 
coloured in every position of op, the colour in one quadrant of 
its circular motion being complementary to that in the adja- 
cent quadrants. If we now wish to compare the quantity of 
polarised light in a pencil reflected at an angle of 8o° from 
silver, with the quantity polarised at the same angle by steel , 
we have only to transmit the one pencil through m, and the 
other through n, and the intensity of the colours will show 
which of the two contains the greatest quantity of polarised 
light. Or if we diminish the inclination of the steel surface 
P p 2 
