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Now φ = - K (M/r), where K is Newton's constant of gravitation,

and M is the mass of the heavenly body. Thus a displacement

towards the red ought to take place for spectral lines produced

at the surface of stars as compared with the spectral lines of

the same element produced at the surface of the earth, the amount

of this displacement being

image060

For the sun, the displacement towards the red predicted by theory

amounts to about two millionths of the wave-length. A trustworthy

calculation is not possible in the case of the stars, because in

general neither the mass M nor the radius r are known.