INTENSITY OF ORDINARY AND EXTRAORDINARY RAYS. 407 
o! 
! 
is equal to the value of aT We thus ascertain the proportion ae 
" 
and likewise = by observing the azimuth of polarization of the 
(7) 
incident ray, at which the ordinary and extraordinary ray disap- 
pear. If we observe the original azimuth of polarization, at 
which all reflected light is polarized in the plane of incidence, con- 
! 
sequently where R, = p P+s'S =0, we obtain = ; if we observe 
the azimuth of polarization of the reflected ray, when the incident 
ray is polarized in a direction parallel to the plane of incidence, 
where R, = s'S and R, = s8, we have in the tangent of this 
! 
azimuth the value of -, &c. In general when a is the azimuth 
of polarization of the incident ray, tan a = = , and « that of the 
reflected ray, we have 
_ ptana+s'. 
~ p/tana+ s 
If we gradually give a different values, and observe each time 
the « belonging to it, we obtain as many equations, from which 
the proportion of the magnitudes p, p/,s, s' can be ascertained 
with great accuracy. Thus three only of the unknown magni- 
tudes’p, 7, $0... remain unestimated, and these are obtained 
from the three equations which the supposition of the transpa- 
rency has yielded. 
I will illustrate this by an example, based upon observations, 
which I made of the natural surface of cleavage of calcareous 
spar. 
The light was incident at about 45°, and the plane of incidence 
was perpendicular to the surface of the section. I must first ex- 
plain what I mean by positive and negative azimuth of the plane of 
polarization. I suppose the feet to face the crystal, the incident 
plane of vibration being directed forwards, and when the plane 
of polarization is now on my /ef¢-hand I call its azimuth positive ; 
when the plane of polarization is on my right, negative. The 
same consideration applies to the reflected plane of vibration, 
where I again suppose myself lying in it and moving forwards, 
the feet facing the reflecting surface. The azimuth of the plane 
of incidence, formed from the surface of the section, I call plus, 
when supposing myself standing on the surface of the crystal, my 
tan « 
