H. Fizeau

“ Strictly speaking, this number has reference to a vacuum, and ought to be divided by the index of refraction for air; but this index differs so little from unity, that, for the sake of simplicity, the correction, which would not alter the last figure by a unit, may be neglected.
The above quantity being divided by the length of an undulation, will give the displacement of the bands in terms of the breadth of one of them. In fact, for a difference of path amounting to undulations, the system of bands suffer a displacement equal to the breadth of bands. ”

“ With respect to the intensity of light, formidable difficulties had necessarily to be encountered. The tubes, which were of glass and . in diameter, had to be traversed by light along their centres, and not near their sides; the two slits, therefore, had to be placed much further apart than is ordinarily the case, on which account the light would, in the absence of a special contrivance, have been very feeble at the point where the interference bands are produced. ”

“ Of the several hypotheses which have been necessitated by the absence of any definite idea of the properties of luminiferous æther, and of its relations to ponderable matter, not one can be considered as established; they merely possess different degrees of probability. ”

“ In this manner a displacement due to motion would have been detected with certainty, even if it had been accompanied by an accidental displacement due, for instance, to some defect of symmetry in the diameters or orifices of the tubes, whence would result an unequal resistance to the passage of air, and consequently a difference of density. ”

“ If be the required retardation and the length of the column of water traversed by each ray, we have, according to the principles proved in the theory of the interference of light, or Since is only the thirty-three millionth part of , this expression may, without appreciable error, be reduced to If be the index of refraction of water, we have the approximate formula Since each ray traverses the tubes twice, the length is double the real length of the tubes. ”

“ The greatest difficulties were encountered in the annealing of the glass plates of the series; and as perfectly homogeneous plates could not be obtained, it was necessary to employ various compensating expedients, all which will be found described in the memoir itself. ”