264 Scientific Intelligence. 
the same plane, by passing, for example, through the same tourmaline 
plate. This arrangement might, in fact, be that of Fresnel, in which a 
slender beam is reflected from. two glass plates very slightly inclined, 
provided that the light were incident at the polarizing angle of glass. 
pendicular to the length of the glass. Hence, if the vibrations of the 
two polarized pencils are really executed perpendicularly to the plane 
of polarization, or parallel to the length of the glass, (according to the 
arrangement above agreed upon,) they will be propagated with differ- 
ent velocities, and the fringes will be displaced parallel to the length of 
the glass, in a direction which might be inferred from some statements: 
of Sir D. Brewster, but which is quite unimportant to the present pur- 
ose. If, however, on the other hand, the vibrations be executed in the 
plane of polarization, or perpendicular to the length of the glass, the 
two rays will traverse the glass with almost, or quite, the same veloci- 
ties, and the fringes will either not be displaced at all, or to a far less 
amount than in the preceding case. By 
21. On certain cases of Elliptic Polarization of Light by Reflexion; 
by Prof. PowEtt, (Proc. Brit. Assoc., from Athen., Sept. 19, No. 986.) 
—From the principle investigated by Fresnel, that polarized light chan- 
ges its plane, in reflexion, by a certain law dependent on the incidence 
(from transparent media) and the extension of a similar law to reflex- 
ion from a second surface, by Sir D. Brewster, (Phil. Trans. 1830, p- 
' 148,) other formule were obtained by the last named philoso her to ex- 
i imself 
the conditions under which the effect may be sensible. There 
are, doubtless, many cases of thin plates in which elliptic polarization 
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