their terrestrial counterparts that the radial velocities of the stars can be ascertained.
In the case of the sun, the delicacy of these comparative observations may be greatly increased in consequence of the great intensity of the light which is available for spectroscopic dissection. The attainable accuracy of observation is in this case such that even the effect of the ellipticity of the earth's orbit, shown by the consequent slightly variable component velocity of the earth in the direction of the sun, can be determined by the changing positions of the spectral lines. Under these conditions, displacements produced by internal motions also become perceptible, such as may arise from rotation of the sun as a whole, from radial convection currents in the absorbing vapours which produce the dark (Fraunhofer) lines, and possibly also from currents more or less parallel to the sun's surface. Then again, the periods of the vibrating particles which produce the lines — and therefore also the positions of the lines in the spectrum — depend to a small extent upon the pressure to which the gases and vapours are subjected.
Hence, it is only by the most careful selection of the point of observation in the sun that there can be any hope of establishing an absolute agreement in wave-lengths between solar lines and those given by the corresponding terrestrial sources. Dr. St. John, of the Mount Wilson Solar Observatory, has found such a region in the higher strata of the chromosphere, in the neighbourhood of the sun's poles. Rotation and radial convection currents are here ineffective, and the disturbing effect of a difference of pressure or density is practically eliminated by taking the luminous gas at a high elevation; other careful observations have also indicated that there are no systematic surface currents which could materially displace the emission lines of the upper chromosphere. Under these favourable circumstances, in the case of the H and K lines of calcium, Dr. St. John has obtained the first direct proof of the agreement of the solar or cosmic wave-length of a substance with its wave-length in a terrestrial source, to the degree of accuracy attainable with the most powerful modern appliances. In other words, under such widely different conditions, and in regions greatly separated in space, calcium preserves its most characteristic and fundamental property, whereby its molecules show the same set of vibration periods.
