"If an evolutionary process has been going on, which is similar for all stars, there is little doubt that from the bright-line stars down to the solar stars the order has been (1) helium or _Orion_ stars, (2) hydrogen or Sirian stars, (3) calcium or Procyon stars, (4) solar or Capellan stars." My investigations on "The Secular Variation of Starlight" (_Studies in Astronomy_, chap. 17, and _Astronomical Essays_, chap. 12) based on a comparison of Al-Sufi's star magnitudes (tenth century) with modern estimates and measures, tend strongly to confirm the above views. With regard to the 3rd-type stars, such as Betelgeuse and Mira Ceti, Schuster says, "It has been already mentioned that observers differ as to whether their position is anterior to the hydrogen or posterior to the solar stars, and there are valid arguments on both sides." Scheiner, however, shows, from the behaviour of the lines of magnesium, that stars of type I. (Sirian) are the hottest, and type III. the coolest, and he says, we have "for the first time a direct proof of the correctness of the physical interpretation of Vogel's spectral classes, according to which class II. is developed by cooling from I., and III. by a further process of cooling from II."[296] Prof. Hale says that "the resemblance between the spectra of sun-spots and of 3rd-type stars is so close as to indicate that the same cause is controlling the relative intensities of many lines in both instances. This cause, as the laboratory work indicates, is to be regarded as reduced temperature."[297] According to Prof. Schuster, "a spectrum of bright lines may be given by a mass of luminous gas, even if the gas is of great thickness. There is, therefore, no difficulty in explaining the existence of stars giving bright lines." He thinks that the difference between "bright line" stars and those showing dark lines depends upon the rate of increase of the temperature from the surface towards the centre. If this rate is slow, bright lines will be seen. If the rate of increase is rapid, the dark-line spectrum shown by the majority of the stars will appear. This rate, he thinks, is regulated by the gravitational force. So that in the early stages of condensation bright lines are more likely to occur. "If the light is not fully absorbed," both bright and dark lines of the same element may be visible in the same star. Schuster considers it quite possible that if we could remove the outer layers of the Sun's atmosphere, we should obtain a spectrum of bright lines.[298]