308 Prof. Trowbridge and Mr. Hutchins on 



o 



wave-lengths of Angstrom and Thalen as final. One eminent 

 authority speaks of them as the "ne plus ultra" of spectro- 

 scopic 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 ThaleVs work on metallic 

 spectra 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 connexion with spectra of terrestrial elements. 



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

 dence 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 vapour formed in the electric arc 

 or the electric spark coincide with the dark lines of the solar 

 spectrum which is photographed directly above the spectrum 

 of the metal on the same sensitive plate, the evidence is usually 

 considered 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 dark lines in the solar spectrum, not 

 only in exact position but in general grouping and character, 

 the evidence cannot be doubted by any one who has carefully 

 examined it. When a majority of the lines of any metal 

 coincide 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 spec- 

 trum, there is a probability that the corresponding lines 

 wanting in the sun have been obliterated by superposed lines 

 or bands of other metals. In our paper u On the Existence 

 of Carbon in the Sun," we shall call attention to a case of 

 such obliteration. It is probable, also, that the non-appear- 

 ance of certain lines in the sun may be due to certain con- 

 ditions of temperature. We shall discuss 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, how- 

 ever, if there is any striking peculiarity in the lines of the 



