224. FRESNEL. 
fests itself in a different way from that of difference of 
intensity. The rays subjected to it, for example, always 
give two images in traversing calc spar; but these images 
are each entirely tinted with a bright and uniform colour. 
Thus, though the incident light may be white, the ordi- 
nary ray may be entirely red, orange, yellow, green, blue, 
or violet, according to the direction in which the prin- 
cipal section of the crystal cuts the ray: and as to the 
extraordinary ray, it will not suffice to say that it never 
resembles the ordinary; we must say that it differs from 
it as widely as possible; that if the one, for example, is 
coloured red, the other shows a bright green, and so on 
for the rest of the prismatic tints. | 
When this new kind of polarized rays are reflected 
from a transparent mirror, we perceive other phenomena 
not less curious. Let us conceive, in fact, to fix the ideas, 
that one of these rays be vertical, and that it fall on a re- 
flector of pure glass at an angle of about 35°, this mirror 
may be on the right side of the ray: and the inclination 
remaining constant, it may be turned to its left, before it, 
or behind it, or in any intermediate position. We may 
remember that the incident ray was white; then, in any 
of these positions of the glass reflector, the ray will not 
have this colour: it will be now red, now orange, yellow, 
green, blue, indigo, violet, according to the s¢de on which 
the glass presents itself to the incident ray ; it is, in fact, 
precisely in this order that the tints succeed one another, 
both the images will be coloured, and their tints complementary. The 
originally polarized light is divided again into two oppositely polarized 
pencils in passing through the film, or as Professor J. Forbes has termed 
it, Dipolarized; others had termed it DEpolarized. This is what Arago 
here calls a new and entirely distinct kind of polarization; though the 
term is, perhaps, not very happily applied. This is what was explained 
at large in a previous note. 
