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Instead we should ascribe the seasonal differences to a meridional movement of the sun. Our spectroscopic astronomy so far as it refers to the composition of the sun and moon would stand precisely where it does, but the bulk of our mathematical astronomy would not exist.
Fortunately, radio-activity can be detected and measured by electrical methods of extraordinary fineness, methods the delicacy of which very much more exceeds that of spectroscopic methods than the sensitiveness of these surpasses that of ordinary chemical analysis.
Since no universally valid measures can be made in any others, their production is quite indispensable to spectroscopic science.
Could the spectroscope be used in determining also the character of the materials in those orbs that we see shining in the depths of space? The instrument was turned in answer to these questions to the sidereal heavens. No other branch of science has been prosecuted in the after half of this century with more zeal and success than has the spectroscopic analysis of the fixed stars.
Yet the main track is plainly continuous, and leads by insensible gradations from nebulæ through helium stars to the Sirian, and onward to the solar type, whence, by an inevitable transition, fluted, or "Antarian," spectra develop. The first-known examples of the class of gaseous stars Beta Lyræ and Gamma Cassiopeiæ were noticed by Father Secchi at the outset of his spectroscopic inquiries.
The existence of an atmosphere charged with vapor has been confirmed also by spectroscopic observations, principally those of Vogel, according to which this atmosphere must be of a composition differing little from our own, and above all, very rich in aqueous vapor.
The components are approximately equal, both in mass and light, and the system formed by them is transported towards us with a speed of some sixteen miles a second. The line-shiftings so singularly communicative proceed, in this star, with perfect regularity. This new class of "spectroscopic binaries" could never have been visually disclosed.
Green is one of the rarest colors among the stars. Castor belongs to the same general spectroscopic type in which Sirius is found, but its lines of hydrogen are broader than those seen in the spectrum of the Dog Star. There is reason for thinking that it may be surrounded with a more extensive atmosphere of that gaseous metal called hydrogen than any other bright star possesses.
In what precedes, the presence of an atmosphere has been assumed, and, fortunately, there is very convincing evidence, both visual and spectroscopic, that Venus is well and abundantly supplied with air, by which it is not meant that Venus's air is precisely like the mixture of oxygen and nitrogen, with a few other gases, which we breathe and call by that name.
Then the spectroscopic principle already mentioned is invoked for studying the shift of the lines, which is toward the violet in the stars ahead of us and toward the red in those that we are leaving behind. Of course the effects of the independent motions of the stars must be carefully excluded.
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