116 MEMOIRS OF TUE NATIONAL ACADEMY OP SCIENCES. 



entered could be regulated by the oiiacity of the paper selected for the screen, and the quantity of 

 gaslight by the number of glass plates used in the retlector before the slit. 



1. By means of the movable screens it was easy to adjust the apparatus so that the quantity 

 of light from the two sources should be sensibly equal; then, when the two small screens before 

 the slit were removed, the spectrum was that of sunlight mixed with an equal (juantity of light 

 yielding a continuous spectrum. This was found to show in the central portion of the spectrum 

 the lines U, E, b, F, and very many distinct but fainter lines in the green. This proves that in 

 the corona less than half the light giving the continuous spectrum is retlected suidight. 



2. The relative quantity of light from the tiame was successively increased. When two or 

 tliree times as great as the proportion of sunlight the fainter lines disappeared and onlj' those 

 remained which are named above ; then, still increasing the relative amount of gas-light, F, E, and 

 D disai)peared, leaving b distinctly visible long after. This can hardly excite surprise, for the h 

 group is obviously the strongest line in a faint solar spectrum. But the necessary deduction from 

 the experiment is a very important one. Since there was enough sunlight in the-coronal spectrum 

 to render visible the weaker line D while the b group was invisible, there must have been present 

 an excess of light of the wavelengths of the b group ; in other words, if the corona had been 

 deprived of all purely solar light the brightest of the magnesium lines would have been added to 

 those of hydrogen and 1474K. 



II. Polarization of the Coronal Light. 



It is unnecessary to review the observations on this point with such minuteness as we have 

 employed in discussion of the spectroscopic evidence. This is owing not only to the thorough 

 manner in which Mr. Eanyard has done it, but also because there is no room for doubt, esxiecially 

 since Professor "Wright's admirable work at the eclipse of 1878, that the coronal light is strongly 

 polarized radially. The intensity of polarization and its distribution with i-eference to the sun 

 are, however, of the greatest theoretical importance. Thus all evidence bearing on these points 

 deserves our critical examination. Unfortunately, there are few observations to guide us here, 

 though some are of great value. There are certain observei's who have given estimates of the 

 percentage of polarization in the coronal light or of its variation in passing from the moon's limb 

 outwards. They are Prazmowski ( 1860), Langley (1870), Blaseena (1870), and Hastings (1878). 

 Mr. Prazmowski writes (p. 281), as nearly as I can recall, the portion of the corona most " stronglj^ 

 colored did not correspond to the most brilliant part, but was situated at a certain distance froin 

 the edge of the moon." This, as a mere recollection, and differing from all others who have used 

 the same method (notably Eanyard, in 1871), has necessarily little weight. 



Professor Langley (U. S. Coast Survey Report, 1S70, pp. 158-164) concluded that the polari 

 zation diminished near the moon's limb, because the bands in his Savart i)olariscope did extend 

 to that limit. But from the i^eculiarity of the api)aratus this conclusion does not follow. The 

 bands were about 1° 50' apart, v.-hile the apparent diameter of the moon was oidy 6° in his 

 instrument, so that successive bauds would fall on widely ditt'ereut regions of the corona. In 

 short, even if the polarization had been 30 per cent, at the limb, the bands, other than the 

 central one, would have taded out before reaching the limb. * 



Professor Blaseena, in the eclipse of 1870 (j). 309), estimated the strength of the polarization 

 near the limb as equal to that of a clear sky at 50° from the sun. This corresponds to a per- 

 centage of about 30. 



Dr. Hastings (U. S. Naval Obs. Report, 1878, p. 158), by an apparatus which did not admit of 



* The theory of this instrument has been discussed more fully in the U. S. N. Report, 1878, p. 161, by the writer. 



