Trowbridge and Hutchins — Oxygen in the Sun. 269 



thority speaks of them as the " ne plus ultra " of spectroscopic 

 accuracy ; and any attempt to revise or correct them may be 

 looked upon as presumptuous. However, we believe the time 

 has arrived when the whole of Thalen's work on metallic spec- 

 tra must be re-examined. It is safe to say that he has tabulated 

 not more than one line in many metals where several exist, and 

 his positions are occasionally wrong by as much as two wave- 

 lengths. 



As yet no approach to the accuracy with which the solar 

 spectrum has been delineated has been attempted in metallic 

 spectra, — a remarkable fact, when we consider that the chief 

 interest that attaches to the study of the solar spectrum is in its 

 connection with spectra of terrestrial elements. 



The test of the existence of oxygen in the sun is the coin- 

 cidence of the bright lines of the spectrum of oxygen with 

 bright lines or with dark lines of the solar spectrum. If the 

 bright lines of any metallic vapor formed in the electric arc or 

 the electric spark coincide with the dark lines of the solar spec- 

 trum which is photographed directly above the spectrum of the 

 metal on the same sensitive plate, the evidence is usually con- 

 sidered conclusive in regard to the existence of the metal in the 

 sun. In the case of iron, where hundreds of lines of the metal 

 coincide with the dark lines in the solar spectrum, not only in 

 exact position but in general grouping and character, the evi- 

 dence cannot be doubted by any one who has carefully ex- 

 amined it. When a majority of the lines of any metal coin- 

 cide with dark lines in the solar spectrum under high dispersion, 

 not only in position but in grouping, while a few of the metal 

 lines have no representatives in the solar spectrum, there is a 

 probability that the corresponding lines wanting in the sun 

 have been obliterated by superposed lines or bauds of other 

 metals. In our paper " On the Existence of Carbon in the 

 Sun," we have called attention to a case of such obliteration. 

 It is probable, also, that the non-appearance of certain lines in 

 the sun may be due to certain conditions of temperature. We 

 have discussed this point more fully in the paper on carbon, 

 above referred to. 



The same remarks apply to the coincidence of the lines of 

 any element with the supposed bright spaces in the sun. The 

 value of the test of coincidence increases with the number of 

 coincidences. If an element has only two or three lines, and 

 these two or three agree in position with dark lines in the solar 

 spectrum, the evidence of the existence of the element in the 

 sun is not conclusive. It is supported, however, if there is any 

 striking peculiarity in the lines of the element which is repro- 

 duced in the corresponding lines in the solar spectrum. Thus 

 the nebulous character of the lines of magnesium is perfectly 



