WHEN EXPOSED TO POLARIZED LIGHT. 
47 
Values of m'. 
m 
Values of , , 
V m!* — 
1-800 
1-202 
1-700 
1-236 
1-600 
1-281 
1-554 
1-307 
1-508 
1-336 
1-500 
1-341 
1-400 
1-428 
1-336 
1-508 
1-307 
1-554 
The limits, therefore, between which the second disappearance of the rings can take 
place are T554, the index for quartz and flint glass, and T307, the index for ice. 
But though the range is very limited, it nevertheless includes a considerable variety 
of solid and fluid bodies. I have omitted the indices of Tabasheer, and the fluids 
produced by the compression of gaseous bodies, because, though their refractive 
powers are beneath T30 7, they cannot be used in the present inquiry. 
When m and m! are thus related, the white centred rings will just disappear when 
i — 90°, the light being then incident on the second surface at its polarizing angle. 
But if we use a film of still less refractive power in relation to the second body, the 
refracted rays will fall on the second surface at an angle greater than the polarizing 
angle (i being still 90°), and consequently the black centred rings will reappear, and 
there will be some angle of incidence I on the film, less than 90°, at which the angle 
of refraction i! will be equal to the polarizing angle of the second surface. This 
angle will be found from the expression 
. T mm' 
sin i = - T - 
When m — m! no rings whatever will be formed, as no light is reflected at the com- 
mon surface ; but if m — m! only for a particular colour in the spectrum of each sub- 
stance, and if these indices differ considerably for another colour, rings will be formed 
in which that colour predominates, in which m > m!, or m < m'. This takes place in a 
remarkable manner with oil of cassia and flint glass, in which m = m! for the red 
rays, but m > m for the blue rays. The consequence of this is, that a quantity of blue 
light is reflected from the separating surface of the oil and the glass ; and hence if a 
sufficiently thin film of oil of cassia is laid upon the glass, blue would greatly predo- 
minate in the system of rings. 
Hitherto the azimuth of the polarized light has been 90°, or perpendicular to the 
plane of reflexion. Let us now suppose that its azimuth is gradually changed from 
90° to 0° by the rotation of the polarizing surface or crystal. 
At all azimuths, from 90° to 0°, the rings with the black circumference are seen, 
between the angles of 0° and 53° 11', and at the incidence of 53° 11'. But at inci- 
