John Strutt, 3rd Baron Rayleigh

“ But if the board be turned through a right angle so as to point east and west, both directions of vibration for light passing the tube are transverse to the earth's motion, and therefore no double refraction could manifest itself. The question is whether turning the board from the north and south position to the east and west position makes any difference. In no case is any effect to be expected from a rotation through 180°, and such effect as a rotation through 90° may entail must be of the second order in the ratio which expresses the velocity of the earth relatively to that of light. ”

“ If this retardation amounted to 1⁄2λ, λ being the wave-length, the effect would be the same as of a rotation of the nicol through 90°. In general, a retardation of phase ε, in place of π, gives a revival of light measured by sin2 (1⁄2ε) . If the revivals of light in the two cases be the same, we may equate θ to 1⁄2ε. Hence if we find that rotation θ produces a sensible effect in lessening the darkness at the darkest place, we may infer that there is delicacy sufficient to detect a relative retardation of 2θ due to double refraction. ”

“ With the water the delicacy of the test was somewhat less.
When it is attempted to replace the liquid by solid matter, the difficulties of experiment are greatly increased. The best results that I have been able to obtain were with built up thicknesses of plate-glass. A sufficient thickness in one piece is liable to exhibit too much double refraction from the effect of internal strains. ”