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We refer to the yearly movement of the apparent position of the

fixed stars resulting from the motion of the earth round the sun

(aberration), and to the influence of the radial components of

the relative motions of the fixed stars with respect to the earth

on the colour of the light reaching us from them. The latter

effect manifests itself in a slight displacement of the spectral

lines of the light transmitted to us from a fixed star, as

compared with the position of the same spectral lines when they

are produced by a terrestrial source of light (Doppler

principle). The experimental arguments in favour of the

Maxwell-Lorentz theory, which are at the same time arguments in

favour of the theory of relativity, are too numerous to be set

forth here. In reality they limit the theoretical possibilities

to such an extent, that no other theory than that of Maxwell and

Lorentz has been able to hold its own when tested by experience.