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Gravitation and Light


As I said last week (p. 334), and also in the December Phil. Mag. (p. 737), the refractivity μâ1, necessary at every point of a gravitational field to produce the Einstein deflection, is the ratio of the energy of a constant-mass particle fallen there from infinity to the energy of the same particle moving with the speed of light; but it is not permissible to say that the solar gravitational field acts like a lens, for it has no focal length. If the sun were backed by a nebula or any luminous area, the light grazing the rim all round would be brought to a focus at a place seventeen times the distance of Neptune, while light from any larger circle would focus still further off in proportion to the area of the circle. So from a uniformly luminous area there would result a focal line of constant brightness. The moon is, unfortunately, impotent to make an annular eclipse interesting.

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LODGE, O. Gravitation and Light. Nature 104, 354 (1919).

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