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NOTE: degrees A (Absolute?) is the same as the currentdegrees K (Kelvin).
THE POPULAR SCIENCE MONTHLY VOLUME LXXXVI JULY TO SEPTEMBER,1915
EDITED BY J. McKEEN CATTELL
THUS far our description of the stellar universe has beenconfined to its geometrical properties. A serious study of theevolution of the stars must seek to determine, first of all,what the stars really are, what their chemical constitutionsand physical conditions are; and how they are related to eachother as to their physical properties. The application of thespectroscope has advanced our knowledge of the subject by leapsand bounds. This wonderful instrument, assisted by thephotographic plate, enables every visible celestial body towrite its own record of the conditions existing in itself,within limits set principally by the brightness of the body.Such records physicists have succeeded to some extent induplicating in their laboratories; and the known conditionsunder which the laboratory experiments have been conducted arethe Rosetta Stones which are enabling us to interpret, withmore or less success, the records written by the stars.
It is well known that the ordinary image of a star, whetherformed by the eye alone, or by the achromatic telescope and theeye combined, contains light of an infinite variety of colorscorresponding, speaking according to the mechanical theory oflight, to waves of energy of an infinite variety of lengthswhich have traveled to us from the star. In the point image ofa star, these radiations fall in a confused heap. and theobserver is unable to say that radiations corresponding to anygiven wave-lengths are present or absent. When the star's lighthas been passed through the prism, or diffracted from thegrating of a spectroscope, these rays are separated one fromanother and arranged side by side in perfect order, ready forthe observer to survey them and to determine which ones arepresent in superabundance and which other ones are lackingwholly or in part. The following comparison is a fair one: theordinary point image of a star is as if all the books in theuniversity library were thrown together in a disorderly butcompact pile in the center of the reading room: we could saylittle concerning the contents and characteristics of thatlibrary; whether it is strong in certain fields of humanendeavor, or weak in other fields. The spectrum of a star is asthe same library when the books are arranged on the shelves incomplete perfection and simplicity, so that he who looks mayappraise its contents at any or all points. Let us consider thefundamental principles of spectroscopy.
1. When a solid body, a liquid, or a highly-condensed gas isheated to incandescence, its light when passed through aspectroscope forms a continuous spectrum: that is, a band oflight, red at one end and violet at the other, uninterrupted byeither dark or bright lines.
2. The light from the incandescent gas or vapor of a chemicalelement, passed through a spectroscope, forms a bright-linespectrum; that is, one consisting entirely of isolated brightlines, distributed differently throughout the spectrum for thedifferent elements,