[659] Ibid., vol. lviii., p. 370.
[660] Astr. and Astrophysics, vol. xi., p. 615.
[661] Thollon's estimate (Comptes Rendus, t. xcvii., p. 902) of 300,000 kilometres, seems considerably too low. Limiting the "average prominence region" to a shell 54,000 miles deep (2′ of arc as seen from the earth), the visual line will, at mid-height (27,000 miles from the sun's surface), travel through (in round numbers) 320,000 miles of that region.
[662] Liveing and Dewar, Phil. Mag., vol. xvi. (5th ser.), p. 407.
[663] Chemistry of the Sun, p. 260.
[664] Nature, October 14, 1886.
[665] The normal spectrum is that depending exclusively upon wave-length—the fundamental constant given by nature as regards light. It is obtained by the interference of rays, in the manner first exemplified by Fraunhofer, and affords the only unvarying standard for measurement. In the refraction spectrum (upon which Kirchhoff's map was founded), the relative positions of the lines vary with the material of the prisms.
[666] Scheiner, Die Spectralanalyse der Gestirne, p. 168.
[667] Phil. Mag., vol. xxvii., p. 479.
[668] Astr. and Astrophysics, vol. xii., p. 321; Frost-Scheiner, Astr. Spectr., p. 363.