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310 THE ADVANCE OF PHOTOGRAPHY
This " a " band is reinforced in the spectrum of Mars,, due of course to absorption taking place in its atmosphere, and since this band in the spectrum of the moon has been found to be due to absorption brought about by water vapour in the atmosphere of the earth, and it appears identical in the two spectra except in respect to intensity, the reasonable conclusion is that the spectrograph has revealed the presence of water vapour in the atmosphere of Mars.
Spectroscopic Binaries. — It may be mentioned that one of the earliest results of the employment of photographic methods for observing the spectra of the stars was the discovery of a class of double stars (binaries), which have accordingly been designated " spectroscopic binaries/ ' Such binaries are often divided into two classes, (1) those in which the spectra of both stars can be photographed and measured, and (2) those in which only one star issufficiently bright to produce a spectrum which can be photographed.
In the first of these two classes the difference between, the movements of the component stars in the line of sight is indicated by a shift of the lines of the spectra relative to one another.
In the stars of the second class the determination of the orbital motion depends upon the measurement of the variable radial motion of the bright component in the line of sight relative to the earth ; this necessitates that the spectrum of the star shall be compared with the spectrum of some suitable artificial source of light, that the amount of shift of certain lines due to the radial motion may be measured. The first spectroscopic binary to be discovered was f Ursse Majoris. This binary was first proved to be so in 1889 ; it belongs to the first-mentioned class. In 1890 Dr Vogel announced that the star Spica (a Virginis) is a spectroscopic binary of the second class.