Existence of certain Elements in the Sun. 327 



the metal under experiment was then placed in the lamp, and 

 the spectrum photographed on the lower part of the plate. 

 Any lines due to impurities would then extend entirely across 

 the plate, while those of the pure metal w r ould extend only 

 half-way. In addition to this precaution we consulted all 

 accessible tables and plates as to the position of known lines 

 of metallic spectra, and also compared together all our photo- 

 graphs of the same region. If all of these tests left any 

 doubt as to the origin of a given line, it was at once subjected 

 to special investigation until all doubt was removed. 



The dispersion given by the apparatus in the order of 

 spectrum in which we worked is such that a single wave- 

 length occupies on the negative a space of 1*12 mm. This 

 makes the distance between the lines D 1 and D 2 67 mm., 

 while the length of spectrum from A to H is about 4'1 m. 

 With so great dispersion it would hardly be possible to mis- 

 take the position of a line by any very considerable amount, 

 or to confound neighbouring lines belonging to different 

 metals. 



For reasons readily apparent, it was found so difficult to 

 photograph under high dispersive power those parts of the 

 spectrum not lying between wave-length 3600 and wave- 

 length 5000, that our photographic work was done chiefly 

 within those limits. It was, however, supplemented in many 

 cases by eye observations in other portions of the spectrum. 



We are convinced that there is much in the whole matter 

 of coincidences of metallic and solar lines that needs re- 

 examination ; that something more than the mere coincidence 

 of two or three lines out of many is necessary to establish 

 even the probability of the presence of a metal in the sun. 

 With the best instruments the violet portion of the solar 

 spectrum is found to be so thickly set with fine lines that, 

 if a metallic line were projected upon it at random, in many 

 places the chances for a coincidence w r ould be even, and coin- 

 cidences could not fail to occur in the cases of such metals as 

 cerium and vanadium, which give hundreds of lines in the 

 arc. 



Moreover, a high dispersion shows that very few lines of 

 metals are simple and short, but, on the contrary, winged and 

 nebulous, and complicated by a great variety of reversal 

 phenomena, A "line" is sometimes half an inch wide on 

 the photographic plate, or it may be split into ten by reversals. 



At first we believed that these reversals were due to defects 

 in the ruling of the grating, but we are convinced that thev 

 are true phenomena from the following experiments: — (1) The 

 wings continue when various portions of the grating are 



