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THE POPULAR SCIENCE MONTHLY. 



seen at a glance. The matching and 

 identification are even more complete 

 than they were in the original experi- 

 ments of Kirchhoff with the metals, for 

 here it is not necessary to invoke a 

 theory for the unification of bright and 

 dark lines ; the bright lines of the spec- 

 trum of oxygen being continuous with 

 the bright lines of the solar spectrum. 

 It is, indeed, because the solar oxygen 

 reveals itself by bright lines that these 

 have not been earlier detected, as they 

 have been masked and concealed among 

 the unoccupied luminous spaces, be- 

 tween the dark lines that have hitherto 

 been the main objects of attention. 



Dr. Draper has been occupied for 

 several years with this investigation 

 in fact, he has grown into it. Besides 

 Lis inherited ai)titude, and life-long 

 training in this delicate line of manip- 

 ulation, and his thorough familiarity 

 with the peculiar difficulties of these 

 investigations, his work could only have 

 become successful by means of a com- 

 bination of appliances, some of which 

 are only lately available. His task was 

 to produce a gas spectrum, and main- 

 tain it at a brilliancy which would ad- 

 mit of its being photographed alongside 

 of that of the sun itself. Oxygen is 

 made incandescent by electricity. The 

 most ample, steady, and sustained com- 

 mand of this agent was therefore in- 

 dispensable. This was secured by the 

 Gramme machine, a dynamo-electric 

 engine connected with a large induc- 

 tion-coil and a battery of Leyden-jars. 

 The impulse was furnished by a Bray- 

 ton's petroleum-motor, which " can be 

 started with a match, comes to its regu- 

 lar speed in less than a minute, and 

 preserves its rate entirely unchanged 

 for hours together." This was belted 

 to the Gramme machine, which, at its 

 usual rate of running, gave 1,000 ten- 

 inch sparks per minute. This " torrent 

 of intense electric fire," consisting of 

 twenty ten-inch sparks per second, was 

 passed through Pliicker's tubes, con- 

 taining oxygen, the spectrum of which 



is thrown upon a sensitive photographic 

 surface, while the solar spectrum is 

 formed beside it, and both are fixed 

 together upon the tablet. The embar- 

 rassments of the investigation are thus 

 referred to in Dr. Draper's paper : 



" This research has proved to be more 

 tedious and difficult than would be supposed, 

 because so many conditions must conspire 

 to produce a good photograph. Tliere must 

 be a uniform, prime-moving engine of two- 

 horse power, a dynamo-electric machine 

 thoroughly adjusted, a large EuhmkorfF coil 

 with its Foucault break in the best order, a 

 battery of Leyden-jars carefully propor- 

 tioned to the Pliicker's tube in use, a helio- 

 stat which of course involves clear sunshine, 

 an optical train of slit, prisms, lenses, and 

 camera well focused, and, in addition to all 

 this, a photographic laboratory in such com- 

 plete condition that wet, sensitive plates can 

 be prepared which will bear an exposure of fif- 

 teen minutes and a prolonged development. 

 It has been difficult to keep the Pliicker's 

 tubes in order; often before the first expos- 

 ure of a tube was over, the tube was ruined 

 by the strong Leyden sparks. Moreover, to 

 procure tubes of known contents is trouble- 

 some. For example, my hydrogen-tubes 

 gave a spectrum photograph of fifteen lines, 

 of which only three belonged to hydrogen. 

 In order to be sure that none of these were 

 new hydrogen-lines, it was necessary to try 

 tubes of various makers, to prepare pure 

 hydrogen and employ that, to examine the 

 spectrum of water, and finally to resort to 

 comparison with the sun." 



In regard to the significance of the 

 inquiry in relation to spectroscopic 

 study, Dr. Draper remarks : 



" We must, therefore, change our theory 

 of the solar spectrum, and no longer regard 

 it merely as a continuous spectrum with 

 certain rays absorbed by a layer of ignited 

 metallic vapors, but as having also bright 

 lines and bands superposed on the back- 

 ground of continuous spectrum. Such a 

 conception not only opens the way to the 

 discovery of others of the non-metals, sul- 

 phur, phosphorus, selenium, chlorine, bro- 

 mine, iodine, fluorine, carbon, etc., but also 

 may account for some of the so-called dark 

 lines, by regarding them as intervals be- 

 tween bright lines." 



