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A few years later Hubble showed, by a special investigation of

the extra-galactic nebulae ("milky ways"), that the spectral

lines emitted showed a red shift which increased regularly with

the distance of the nebulae. This can be interpreted in regard to

our present knowledge only in the sense of Doppler's principle,

as an expansive motion of the system of stars in the large-as

required, according to Friedman, by the field equations of

gravitation. Hubble's discovery can, therefore, be considered to

some extent as a confirmation of the theory.