Albert A. Michelson

“ If the earth were a transparent body, it might perhaps be conceded, in view of the experiments just cited, that the inter-molecular æther was at rest in space, notwithstanding the motion of the earth in its orbit ”

“ It is evident that light from will form a stationary image at and similarly light from will form a stationary image at . If now the velocity of the mirrors be increased sufficiently, their phases still being exactly the same, both images will be deflected from and , in inverse proportion to the velocities of light in the two directions; or, if the two deflections are made equal, and the difference of phase of the mirrors be simultaneously measured, this will evidently be proportional to the difference of velocity in the two directions. ”

“ If now the whole apparatus be turned through , the difference will be in the opposite direction, hence the displacement of the interference-fringes should be . Considering only the velocity of the earth in its orbit, this would be . If, as was the case in the first experiment, waves of yellow light, the displacement to be expected would be of the distance between the interference fringes. ”

“ The arrangement of apparatus might be as in fig. 2 (Figure 07) ; , in the focus of the lens , is a slit. are two glass mirrors optically plane and so silvered as to allow say one twentieth of the light to pass through, and reflecting say ninety per cent. The intensity of the light falling on the observing telescope would be about one millionth of the original intensity, so that if sunlight or the electric arc were used it could still be readily seen. The mirrors and , would differ from parallelism sufficiently to separate the successive images. ”