KNOWLEDGE 



[Septembee 1, 1891. 



depend mainly upon their powers of flight for obtaining 

 their food, most of them only make use of the surface of 

 the water, upon which they float placidly, as a resting- 

 place. (To lie continued.) 



Prism breaking uji a ray of white 

 from S. into a spectrum R.T. 



ON THE RHYMICAL GROUP OF HYDROGEN 

 LINES VISIBLE IN MANY STELLAR SPECTRA. 



By A. C. Hanyasd. 



AS all readers of Knowxedge know, a prism or wedge- 

 shaped piece of glass breaks up a beam of white 

 light into a series of coloured beams arranged 

 fan-wise, the blue or nolet rays being turned 

 through the greatest angle. If such a prism be 



placed in front 

 of a telescope, 

 so as to send 

 the refracted 

 rays from a 

 star down the 

 axis of the 

 telescope, an 

 observer look- 

 ing in at the 

 eye end will 

 see the image 

 of the star 

 spread out into 

 a narrow line 

 of light, blueat 



one end and red at the other. This narrow band or spectrum 

 is not continuous, but is interrupted by small dark gaps, 

 which are not very easy to see — but if the narrow spectral 

 band of light is made to appear a little broader by looking 

 at it with a cylindrical lens, the dark gaps become visible 

 as lines across the spectrum, and appear similar to the 

 dark lines across the Stellar spectra shown on the plate. 



The spectra reproduced in the plate were, however, 

 photographed without a cylindrical lens. The narrow 

 spectral streak of coloured light was turned so as to be at 

 right angles to the direction of the star's diurnal motion, 

 and by slowing the driving clock of the telescope the 

 spectral streak was caused to sweep across the sensitive 

 plate in a direction at right angles to its length, thus 

 producing a trail or trace with dark lines across it, 

 corresponding to the dark gaps in the spectral streak. The 

 upper four Stellar spectra on the plate are from photo- 

 graphs kindly given me by the Brothers Henry, and the 

 lower fom' are copied from photographs published by 

 Prof. E. C. Pickering in connection with the spectro- 

 scopic work of the Draper memorial. 



The method of observing Stellar spectra with a prism in 

 front of the object-glass, and a cylindrical lens, was devised 



by Fraunhofer — nearly 

 80 years ago — in 1814. 

 This remarkable man* 

 turned a telescope with 

 a prism in fiont of the 

 object-glass to the stars, 

 and noted some of the 

 chief variations in their 

 spectra. He found that 

 the spectrum of Pollux 

 closely resembled the 

 spectrum of our Sun, 

 while there were re- 

 cognizable differences 



Prism in front of Object-glas; 



* Fraimhofer greatl_v improved the achromatic telescope. He 

 succeeded, after many eiperiments, in making the first large achromatic 



in the spectra of Capella, Betelgeuse and Procyon- — they 

 were all crossed by narrow dark lines, some of which he 

 identified with solar lines. On the other hand, the spectra 

 of Sirius and Castor were seen to be of a different type, 

 and to be crossed by three massive dark bars, two in the 

 blue .and one in the green. 



After a lapse of 45 years Prof. Kirchhoff, of Heidelberg, 

 found a key which enabled him partially to read the hiero- 

 glyphic language of the Fraunhofer lines, but much more 

 remains still undeciphered until some Daniel shall arise 

 who can intei-pret the story clearly written in letters of 

 light upon the heavens. Kirchhoff passed a beam of sun- 

 light across a space occupied by burning sodium vapour, 

 and perceived with astonishment that the dark Fraunhofer 

 line D, instead of being blotted out by the luminous rays 

 of the same refrangibility as that given out by the flame, 

 were rendered blacker and thicker by the superposition. 

 He tried the same experiment, substituting the continuous 

 spectrum derived from the light of a Drummond lamp for 

 sunlight, but a dark line, corresponding in every respect 

 to the solar D line, was seen to cross the spectrum. The 

 inference was irresistible that these dark lines were pro- 

 duced by absorption of light corresponding in wave-length 

 to the bright lines given out by the vapour, and that there 

 must be an absorbing layer ol sodium vapour about the 

 Sun. This discovery was quickly followed up by the 

 examination in the laboratory, by Prof. Kirchhoff, of the 

 bright lines from other incandescent metallic vapours, and 

 many of the bright lines from such vapours were identified 

 with dark lines in the Solar spectrum. But in spite of all 

 the work done by Prof. Kirchhoff' and his assiduous pupil, 

 Dr. Bunsen, and in sjjite of all the careful measuring and 

 photographing of the lines in the spectra of terrestrial 

 elements since their day, not one-fourth of the dark lines 

 of the Solar spectrum have as yet been identified with 

 lines in the spectra of terrestrial elements ; and there are 

 many mysterious discrepancies between the spectra of the 

 elements, as observable under laboratory conditions, and 

 the corresponding dark hues of the Solar spectrum. We 

 are, therefore, very far from having read the riddle of the 

 Solar spectrum, and we are much further fi'om having 

 learnt aU that may be deciphered from the spectra of the 

 stars. 



Soon after the publication of Kirchhofi''s and Bunsen's 

 results. Father Secchi set himself to the task of making a 

 spectroscopic survey of the heavens. He examined the 

 spectra of 4000 stars, and grouped them into four great 

 classes, with the first of which we are at present more 

 especially concerned. It contained Sirius and Vega, and 

 more than half of the stars, which he examined. Their 

 spectra are characterized by the great strength of the 

 hydrogen lines, which are wide, hazy bands, much like 

 the H and K of the Solar spectrum, though these lines in 

 the Solar spectrum do not belong to hydrogen but to 

 calcium, an element which is closely associated with 

 hydrogen in the Solar prominences, but it is very 

 different from it in atomic weight and chemical qualities. 



object-glass of high quality and finish. Tt was secured by the elder 

 Strave for the Russian Grorernment, and was long known as the " great 

 Dorpat refractor," though it was only 9\ inches in diameter, and was 

 of 14 feet focal length. He discovered nearly a thousand lines in 

 the Solar Spectrum, and mapped 576 of them, naming the ])rincipal 

 ones by the letters of the alphabet. He recognized the double D line 

 in many Terrestrial Spectra, and noted the identity of its place with 

 the solar D lines, though the true interpretation of the coincidence 

 was not recognized for more than 40 years after he had laid such 

 ample foundations for the deductions of modem spectrum analysis. 

 After many ingenious experiments, he succeeded in making a 

 diffraction gi-ating, which showed him the lines in the normal 

 spectrum. 



