SOLAK ECLIPSE, MAY G, 1883. 115 



SUMMARY OP SPECTROSCOPIC EVIDENCE. 



This review, in wliicli all the available spectroscopic evidence concerning the natnre of the 

 corona is collected, is most instrnctive. Leaving ont of account tlie integrating spectroscopes, 

 l>ecanse they do not distingnisii between the lines due to the corona alone and those which have 

 their origin in chromospheric light, we find that not a single observer, who has made nse of an 

 instrument which by description is as efiQcient as a single- prism si)e(!troscope of i-inch aperture, has 

 failed to see the bright 1471 K line in the spectrum of the corona. As to the height to which this 

 line could be traced, at no eclipse when the effort has been made has this been less than 25' as a 

 maximum except dnring that of 1878, when a single observer only (Eastman) attempted to find its 

 limits ; he found 14' for a maximum. 



The evidence of the existence of the hydrogen-spectrum in the coronal light i y, really not less 

 conclusive. Ten of the fifteen observers who saw 1474K also saw the hydrogen lines. But what 

 adds indefinitely to the strength of the evidence is the fact that all the observers wbo describe their 

 apparatus as of greater efficiency than a single prism of 'i inch aperture saw the bright hydrogen 

 lines, with the exception of two. Of these two, one (Young, in 18Gfl) may have had too narrow a 

 slit, as he was looking particularly for dark lines ; the other (Eastman, 1878) was observing at a 

 time when all the bright lines were exceptionally feint. The height to which the hydrogen lines 

 have been traced varies from 5' or 6' up to nearly 25'. The line D3 is to be classed with those of 

 Lydrogen. 



All the evidence, some of it most decisive, goes to show that the distribution of the light which 

 yields this line spectrum has no relation to the rifts and streamers which characterize the various 



coronas. 



As to the presence of dark lines in the spectrum of the corona, the evidence is more conflicting. 

 It is evidentthat the integrating spectroscope is, if properly designed, as well suited for the detection 

 of such lines as the analyzing, but with the exception of Brown, in 1883, who reported D as a dark 

 line, no one has detected any with this form of instrument. With analyzing spectroscopes, Janssen, 

 in 1871, saw D with some faint lines in the green, and the writer 1) alone in the recent eclipse. 

 Besides these, a single observer (Barker, in 1878) has reported all the prominent Fraunhofer lines 

 as evident. On the other hand, many of the most experienced observers with the most efficient 

 apparatus have foiled to detect them, even when looking specially for them. The observations of 

 Mr. Stone in 1874 are not included here, as they applied to the outer corona only. Whatever in- 

 terjiretation be placed upon the presence or total absence of dark lines, the conclusion is inevitable 

 that the proportion of true solar light in that of the corona within 5' or G' of the moon's limb is so 

 small that all but the strongest of the Fraunhofer lines are invisible in any spectroscope which lias 

 hitherto been employed. Here wehave a starting point from which we can, by means of experiment, 

 determine tbe minimum limit of the ratio of snnligbt in the corona. It is only necessary to mix 

 light which yields a continuous spectrum with sunlight until we can secure a spectrum which re- 

 sembles that of the corona. 



This experiment I have tried in the following manner : Before the slit of the same spectroscope 

 used at Caroline Island I placed a number of glass plates, varying from one to twelve, at an angle 

 of 450. In front of these was a screen of white paper upon which fell a beam of sunlight from a 

 heliostat. A gas-flame was placed in such a position that a portion of its light was reflected into 

 tlie collimator by the glass plates. Two small movable screens were so adjusted that, at will, one- 

 lialf of the slit could receive sunlight alone, while the other half was illuminated by gaslightonly, 

 or, the whole slit could receive both kinds of light simultaneously. The quantity of sunlight which 



