The method originally used by Huggins, who first conceived and proved the possibility of measuring stellar velocities in the line of sight, was to measure with a filar micrometer the displacement of some well-known line in a stellar spectrum relative to the corresponding line of a terrestrial spectrum.
In the last quarter of the 19th century spectroscopy and photography together worked a revolution in observational astronomy, and in both branches Huggins acted as pioneer.
In solar physics Huggins suggested a spectroscopic method for viewing the red prominences in daylight; and his experiments went far towards settling a much-disputed question regarding the solar distribution of calcium.
This circumstance appeared so anomalous that some astronomers doubted whether the surviving lines were really due to calcium; but Sir William and Lady Huggins (née Margaret Lindsay Murray, who, after their marriage in 1875, actively assisted her husband) successfully demonstrated in the laboratory that calcium vapour, if at a sufficiently low pressure, gives under the influence of the electric discharge precisely these lines and no others.
Sir William Huggins, who was made K.C.B.