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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



been known as an earnest student of science, 

 approached this subject with a wealth of instru- 

 mental means almost beyond precedent; and his 

 well-known skill and assiduity enabled him to ac- 

 cumulate facts of the very greatest importance in 

 the course of the two or three years during which 

 his work was carried on. I am most anxious to 

 make these preliminary remarks, and to state my 

 very highest respect for Dr. Draper, because in 

 going over his work I find that some of his re- 

 sults are, in my opinion, open to doubt. Dr. 

 Draper, in the first instance, apparently unaware 

 of what has hitherto been published with regard 

 to them, announces the discovery of the blight 

 lines already referred to, and, more than this, lie 

 bases a new theory of the solar constitution upon 

 them. It is by no means as a stickler for priority 

 that I regard this as a very great pity; but because 

 I think that if the very considerable literature 

 touching these bright lines — the papers by Young, 

 Cornu, Hennessy, Secchi, and others — had been 

 before Dr. Draper when his memoir was written, 

 the necessity for the establishment of a new the- 

 ory of the solar spectrum, which doubtless cost 

 him very considerable thought, would probably 

 have been less obvious. As a matter of fact, one 

 of the first lines recorded in the spectrum of the 

 chromosphere in 1868 raised the whole question, 

 because there was no dark Fraunhofer line cor- 

 responding with it in the ordinary spectrum of 

 the sun. 



Before I proceed further, it will be best to 

 give some extracts from Dr. Draper's memoir. 

 He writes : 



" Oxygen discloses itself by bright lines or bands 

 in the solar spectrum, and does not give dark ab- 

 sorption-lines like the metals. We must, there- 

 fore, change our theory of the solar spectrum, and 

 no longer regard it merely as a continuous spec- 

 trum with certain rays absorbed by a layer of 

 ignited metallic vapors, but as having also bright 

 lines and bands superposed on the background of 

 continuous spectrum. Such a conception not only 

 opens the way to the discovery of others of the 

 non-metals, sulphur, phosphorus, selenium, chlo- 

 rine, bromine, iodine, fluorine, carbon, etc., but 

 also may account for some of the so-called dark 

 lines, by regarding them as intervals between 

 bright lines. It must be distinctly understood 

 that, in speaking of the solar spectrum here, 1 do 

 not mean the spectrum of any limited area upon 

 the disk or margin of the sun, but the spectrum of 

 light from the whole disk. I have not used an 

 image of the eur. upon the slit of the spectroscope, 

 but have employed the beam reflected from the 

 flat mirror of the heliostat without any con- 

 denser." 



The photograph of the solar spectrum which 

 accompanies Dr. Draper's paper contains a spec- 

 trum of the sun, compared with that of air, and 

 also some of the lines of iron and aluminium. 

 The photograph itself is absolutely free from 

 handwork or retouching. It is, as Dr. Draper 

 points out, difficult to bring out in a single pho- 

 tograph the best points of these various sub- 

 stances. 



" There are so many variables among the con- 

 ditions which conspire for the production of a 

 spectrum that many photographs must be taken 

 to exhaust the best combinations. The pressure 

 of the gas, the strength of the original current, the 

 number of Ley den-jars, the separation and nature 

 of the terminals, the number of sparks per minute, 

 and the duration of the interruption in each spark, 

 are examples of these variables." 



Still, in the particular photograph placed in 

 evidence, Dr. Draper is of opinion that — 



" no close observation is needed to demonstrate 

 to even the most casual observer that the oxygen- 

 lines are found in the sun as bright lines, while 

 the iron-lines have dark representatives." 



And he gives a list of many such coincidences. 



In order to be certain that a line belongs to 

 oxygen, Dr. Draper has compared, under various 

 pressures, the spectra of air, ox3 r gen, nitrogen, car- 

 bonic acid, carburetted hydrogen, hydrogen, and 

 cyanogen. Where these gases were in Fliicker's 

 tubes, a double series of photographs has been 

 needed, one set taken with and the other without 

 Leyden-jars. 



To account for this wonderful discovery com- 

 ing so late, it is urged that — 



" the bright lines of oxygen in the spectrum of 

 the solar disk have not been hitherto perceived, 

 probably from the fact that in eye-observation 

 bright lines on a less bright background do not 

 make the impression on the mind that dark lines 

 do. When attention is called to their presence, 

 they are readily enough seen, even without the 

 aid of a reference spectrum. The photograph, 

 however, brings them into a greater prominence." 



Dr. Draper then passes from facts to theory : 



" From purely theoretical considerations, de- 

 rived from terrestrial chemistry and the nebular 

 hypothesis, the presence of oxygen in the sun 

 might have been strongly suspected, for this ele- 

 ment is currently stated to form eight-ninths of 

 the water of the globe, one-third of the crust of 

 the earth, and one-fifth of the air, and should 

 therefore probably be a large constituent of every 

 member of the solar system. On the other hand, 

 the discovery of oxygen, and probably other non- 

 metals in the sun, gives increased strength to the 



