TRANSACTIONS OF THE SECTIONS. 7 
since been deduced from a very profound mathematical investigation by Mr. Green, 
in the Cambridge Philosophical Transactions, vol. vii—whose results, however, do 
not seem to have met with much attention. Now, however, that they have met with 
the above undesigned general confirmation, it seems very desirable that they should 
be compared with the numerical results of experiments of Sir D. Brewster and Prof. 
Powell. Mr. Green adopts, as part of the basis of his calculation, the original view 
of Fresnel,—that the vibrations of a polarized ray are perpendicular to the plane of 
polarization ; but as this point is a matter of dispute amongst mathematicians, I have 
thought of an experimental method by which this point might, as I think, be decided, 
independently of all theory. Itconsists in the observation of the shifting of the fringes 
produced by two pencils of light polarized in the same plane on interposing in their 
paths a piece of compressed glass. This last apparatus is to be constructed in the 
following manner :—A strip of clear plate, 4 or 5, inches long by half an inch broad, 
is to be provided; and its narrowest faces (or narrowest long sides of the parallelo- 
piped) are to be carefully polished, and rendered perfectly plane and parallel to each 
other,—at least, in the middle part of their length, through which the light is to pass, 
And the glass must be so well annealed and so free from striz as to allow of the for- 
mation of fringes by interfering pencils which have traversed it. It is to be provided 
with a wooden frame and screw, capable of compressing it in the middle, [A similar 
apparatus has already been employed by Brewster, Ling, and Pouillet, to show that 
glass under pressure possesses double refraction.] We may now proceed to the ex- 
periment itself. Let us suppose, then, that the arrangements have been made in a 
darkened room for producing the interference of two pencils of light, which are to be 
polarized in 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. And, for the sake of clearness, let us suppose 
the two foci, or virtual foci, to be vertically one above the other, the plane of polari- 
zation to be vertical, and the glass to be interposed with its length horizontal, Then, 
in its natural state, it will produce no displacement of the fringes, if made carefully 
after the above description. But let us consider what will be its effect if interposed in 
its bent state. The elasticities on its convex and concave sides are different in this 
respect, that the particles are dilated or compressed parallel to the length of the glass ; 
whereas little or no alteration of elasticity is produced in a plane perpendicular 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 different velocities, and the fringes will be displaced paralle] 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 purpose. 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 
velocities, and the fringes will either not be displaced at all, or to a far less amount 
than in the preceding case. 
Notice of a New Property of Light exhibited in the Action of Chrysammate 
of Potash upon Common and Polarized Light. By Sir Davip BREewsTER, 
KL, FRS. . 
The Chrysammate of Potash, which crystallizes in very small, flat rhombic plates, 
has the metallic lustre of gold, whence it derives its name of golden sand. When the 
_ . sun’s light is transmitted through the rhombic plates it has a reddish yellow colour, 
and is wholly polarized in one plane. When the crystals are pressed with the blade 
of a knife on a piece of glass, they can be spread out like an amalgam. The light, 
transmitted through the thinnest films thus produced, consists of two oppositely polar- 
ized pencils,—the one of a bright carmine red and the other of a pale yellow colour, 
With thicker films, the two pencils approach to two equally bright carmine red pencils, 
It is to the reflected light, however, and its new properties, that I wish to direct the 
attention of the Section. Common light, reflected at a perpendicular incidence from 
the surfaces of the crystals, or of the films, has the colour of virgin gold, It grows 
less and less yellow as the incidence increases, till it becomes of a pale bluish white 
