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This process of rolling out involves, it is true, much loss of light a scanty and precious commodity, as coming from the stars; but the loss is an inevitable one. And so fully is it compensated by the great light-grasping power of modern telescopes that important information can now be gained from the spectroscopic examination of stars far below the range of the unarmed eye.
His great map of the "normal" solar spectrum was published in 1868, two years before he died. Robert Thalèn was his coadjutor in its execution, and the immense labour which it cost was amply repaid by its eminent and lasting usefulness. For more than a score of years it held its ground as the universal standard of reference in all spectroscopic inquiries within the range of the visible emanations.
Its period of a little over eleven years is the shortest attributable to a visible binary system, except that of Delta Equulei, provisionally determined by Professor Hussey in 1900 at 5·7 years, and indicated by spectroscopic evidence to be of uncommon brevity. Burnham's Catalogue of 1,290 Double Stars, discovered by him from 1871 to 1899, is a record of unprecedented interest.
Our results were totally negative, but so far as Bonellia was concerned this was not to be wondered at since the later spectroscopic investigations of Sorby and Schenk had fully confirmed the opinion of Lacaze-Duthiers as to the complete distinctness of its pigment from chlorophyl.
The work of each was happily directed so as to supplement that of the other. With less perfect appliances, the Roman astronomer sought to render his extensive rather than precise; at Tulse Hill searching accuracy over a narrow range was aimed at and attained. To Father Secchi is due the merit of having executed the first spectroscopic survey of the heavens.
As to established facts, we learn from the spectroscopic researches that the continuous spectrum is derived from the photosphere or solar gaseous material compressed almost to liquid consistency; that the reversing layer surrounds it and gives rise to black lines in the spectrum; that the chromosphere surrounds this, is composed mainly of hydrogen, and is the cause of the red prominences in eclipses; and that the gaseous corona surrounds all of these, and extends to vast distances outside the sun's visible surface.
Recently, indeed, a somewhat sudden and severe check has been placed on the liveliness of imagination which had enabled men formerly to picture to themselves the inhabitants of other orbs in space. Spectroscopic analysis and exact telescopic scrutiny will not permit some speculations to be entertained which formerly met with favour.
Through his systematic investigations of stellar radial velocities with the Mills spectrograph, knowledge in this department has, since 1897, progressed so rapidly that the spectroscopic binaries of our acquaintance already number half a hundred, and ten times as many more doubtless lie within easy range of detection.
Individual distinctions there had been, but no specific differences. Now all these bodies had kept at a respectful distance from the sun; for of the great comet of 1880 no spectroscopic inquiries had been made.
All the lines equally showed a slight displacement, indicating a recession from the earth of the radiating body at the rate of 37 to 46 miles a second. A similar observation, made by M. Thollon at Nice on the same day, gave emphatic sanction to the spectroscopic method of estimating movement in the line of sight.
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