Another method of obtaining spectra had long been known, by
transmission through, or reflection from, a grating of equidistant
lines ruled upon glass or metal. H. A. Rowland developed the art of
constructing these gratings, which requires great technical skill, and
for this astronomers owe him a debt of gratitude.

In 1842 Doppler[5] proved that the colour of a luminous body, like the
pitch or note of a sounding body, must be changed by velocity of
approach or recession. Everyone has noticed on a railway that, on
meeting a locomotive whistling, the note is lowered after the engine
has passed. The pitch of a sound or the colour of a light depends on
the number of waves striking the ear or eye in a second. This number
is increased by approach and lowered by recession.

Thus, by comparing the spectrum of a star alongside a spectrum of
hydrogen, we may see all the lines, and be sure that there is hydrogen
in the star; yet the lines in the star-spectrum may be all slightly
displaced to one side of the lines of the comparison spectrum. If
towards the violet end, it means mutual approach of the star and
earth; if to the red end, it means recession. The displacement of
lines does not tell us whether the motion is in the star, the earth,
or both. The displacement of the lines being measured, we can
calculate the rate of approach or recession in miles per second.

In 1868 Huggins[6] succeeded in thus measuring the velocities of stars
in the direction of the line of sight.

In 1873 Vogel[7] compared the spectra of the sun's East (approaching)
limb and West (receding) limb, and the displacement of lines endorsed
the theory. This last observation was suggested by Zollner.