APPLICATION TO SPECTROSCOPIC PHENOMENA.
The displacement of the hydrogen line is explained by precisely the same reason ing, only here, much greater velocities are dealt with. Since the intrinsic velocity of light, 186,330 miles per second, is the same for all colours, and the wave-length at the different parts of the spectrum is easily ascertained when using a diffraction grat ing, Doppler's Principle may be used to calculate the velocity of the moving source of light, which, in the case just mentioned, is a mass of glowing hydrogen. For com parison, just under the spectrum of the solar hydrogen is placed the spectrum of terrestrial hydrogen, heated by an electric spark. The displacement of the solar hydrogen line is measured by means' of a micrometer screw, which moves a cross wire in the eyepiece, or, better still, the two are photographed together, and the displacement is measured at leisure with the help of a vernier-reading micro scope. From the displacement, the
change of wave-length is calculated ; this at the indicated parts of the spectrum is as follows :— F line .. 76'66 radial miles per see.
D3'12 C 51.)-78 The velocity obtained is that component which lies in the line of sight. The actual speed may be many times as great, but can never be less (see Fig. 810). The rela tive velocity obtained must be corrected to find that with which the body is mov ing in space. Like most astronomical cal culations this is very complicated and tedious, and includes allowance for the following velocities :—(i.) The earth's diurnal rotation. (ii.) The earth's revo lution about the centre of gravity of the earth and moon. (iii.) The earth's revo lution about the sun. (iv.) In the case of bodies which are entirely outside the solar system, the movement of the solar system in space.