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knowledge of a star’s motion the proper mo-
tion and parallax must also be known.
When Huggins first applied the Doppler
principle to measure velocities in the line
of sight,[7] the faintness of star spectra di-
˜
minished the accuracy; but VAgel, in 1888,
overcame this to a great extent by long ex-
posures of photographic plates.
It has often been noticed that stars which
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