318 ANNUAL OF SCIENTIFIC DISCOVERY. 



the .-liil of Rnthoiford's screw, which the professor pronounced 

 the n)o>t perfect work of art in the worhi. The photograph can 

 be taken in one-thirtieth of a second, and we can get the semi- 

 dianieti r of tiic moon to oni'-tenth of a second of arc. Professor 

 Peirce regards the theory that the corona is of the same charac- 

 ter as the aurora, as the most phiusible one. 



OBSERVATIONS OF THE CORONA DURING THE TOTAL ECLIPSE, 

 AUGUST 7, 1869. BY PROFESSOR EDWARD C. PICKERING. 



Among other expeditions to obsen'e the recent eclipse was one 

 under the direction oi Professor Henry Morton, sent by the Nau- 

 tical-Almanac Office to photograph the sun. I was attached to 

 tliis party to make general and piiysical observations, and from 

 our station at Mt. Phasant, Iowa, arrived at tiie follow ingresidts : — 



It is commonly supposed that the light of the corona is polar- 

 ized in planes passing through the sun's centre, and that it shines 

 by reflected light. Wishing to A'erify this observation, I prepared 

 an Arago's polariscope (in which the ol)jects are viewed through 

 a plate of quartz), and a double-image prism of Iceland spar. 

 1'he two images appear of complementary colors when the light 

 is polarized, the tint changing with the plane of polarization. I 

 therefore expected to see two colored coronas, the tint of each 

 ])ortion being complementary to that of the part at riirht angles to 

 it, and the color revolving with the polarjscope. In reality the 

 two images were pure white, without any traces of color ; but the 

 sky adjoining one was blue, adjoining the other yellow. As the 

 instrument is of consideraljle delicacy, we must conclude that 

 little or no polarized light is emitted by the corona. The sky 

 adjoining it, however, is jjolarized iii a plane independent of the 

 position of the sun, since its color (as seen in the polariscope) is 

 the same whether above, below, or on one side of it. The most 

 ])robttble explanation of this curious phenomenon is, that the 

 earth beycmd the limits of the shadow, being strongly illuminated, 

 acts as a new source of light, and thus gives rise to a polarization 

 in a plane perpendicular to the horizon. 



In hopes of determining the cause of discrepancy between this 

 Qbservation and those previously made, I have endeavored to 

 L'arn what form of polariscope has heretofore been used ; but un- 

 fortunately, in most cases, no description has been published. 

 One observer used a Savart^s polariscope, and, holding it with its 

 j)rincipal plane vertical, found strong traces ot polarization in this 

 l)lane. This observation, however, agrees with mine if we sup- 

 pose that the polarization of the sky was taken for that of the co- 

 rona, a natural mistake with tliis form of instrument. Another 

 observer, wiio used a single plate of tourmaline, saw no evidence 

 of polarization, that of the sky being too feeble to be perceived in 

 this way. I verilied my results with a simple prism of Iceland- 

 epar, with wliich two images of the corona were seen precisely 

 alike, and showing no signs of polari^saiiop, We cannot infer 



