We can not define more clearly the nature of this ether strain or displacement until we know much more about the structure of the ether than we do at present.
"Hertzian Wave Wireless Telegraphy"
John Ambrose Fleming
For this purpose there is made to pass through the spectroscope light from an artificial source which contains one or more chemical elements known to be present in the star which is to be observed, and the corresponding lines in the spectrum of this light and in the spectrum of the star are examined to determine whether they exactly match in position, or show, as they sometimes do, a slight displacement, as if one spectrum had been slipped past the other.
"A Text-Book of Astronomy"
George C. Comstock
This connection between nearness and rapidity of proper motions is indeed what we should expect to find, since a given amount of real motion of the star along its orbit will produce a larger angular displacement, proper motion, the nearer the star is to the earth, and this fact has guided astronomers in selecting the stars to be observed for parallax, the proper motion being determined first and the parallax afterward.
"A Text-Book of Astronomy"
George C. Comstock