Padre Secchi, the Roman astronomer, divides the stars into three types; the first and most dominant type includes Sirius, α Lyræ, and other white stars, which invariably contain hydrogen of high temperature, and are denoted by a black line in their spectra, which coincides with the solar line F; and there is another band also probably due to hydrogen in the violet half of the stars visible to the naked eye belonging to this group. A singular modification of this group, however, occurs in the stars of the constellation Orion, which so rarely show any deviation from one type, that, with the exception of α Orionis or Betelgeux, they may be said to form a family distinguished from all the other stars in the sky; their spectra are crossed by fine lines, faint in the violet, with a band more or less visible in F. γ Cassiopeiæ and β Lyræ differ from the stars of the first type in having a bright band near the solar line F, instead of a black one.[22] Padre Secchi’s second type includes α Orionis, α Tauri, Antares, β Pegasi, &c., which have coloured bands in the red and orange. According to M. Secchi, the most remarkable star in this section is α Herculis. It gives a spectrum which has the appearance of columns illuminated on one side; ‘the stereoscopic effect of the convexity of these bands due to the shading is so surprising, that it cannot be beheld without astonishment.’ The spectrum of the star δ^2 Lyræ has a similar appearance, only instead of convex it has concave bands. The third type consists of stars whose spectra are crossed by fine lines, as Arcturus, Capella and our own sun. The colours of the stars are produced by vapours existing in their atmospheres, one colour predominating over the others, which are absorbed by the number of dark lines. Messrs. Huggins and Miller obtained extraordinary results from the examination of temporary and periodic stars. Temporary stars suddenly shine forth with great brilliancy and soon vanish or nearly vanish. A temporary star which suddenly appeared on the night of May 12, 1866, when examined with a spectroscope, had two spectra, showing that its light emanated from two distinct sources. One spectrum, analogous to that of the sun, was formed by the light of an incandescent solid or liquid photosphere, which suffered absorption by the vapours of an envelope cooler than itself. The second spectrum consisted of a few bright lines, indicating that the light by which it was formed was emitted by luminous gas: the position of some of the lines denoted hydrogen; whence the observers believed the phenomena to result from the burning of hydrogen with some other element, and that the photosphere was heated to incandescence by the resulting temperature.