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XXXIII. Observations of the Corona daring the Total Eclipse, 

 August 7th, 18G9. By Professor Edward C. Pickering*. 



A MONG other expeditions to observe the recent eclipse was 

 -"*• one under the direction of Professor Henry Morton, sent 

 by the Nautical- Almanac Office to photograph the sun. I was 

 attached to this party to make general and physical observations, 

 and from our station at Mount Pleasant, Iowa, arrived at the 

 following results. 



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 verify this observation, I pre- 

 pared an Arago's polariscope (in which the objects are viewed 

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

 spar. The two images appear of complementary colours when 

 the light is polarized, the tint changing with the plane of polar- 

 ization. I therefore expected to see two coloured coronas, the 

 tint of each portion being complementary to that of the part at 

 right angles to it, and the colour revolving with the polariscope. 

 In reality the two images were pure white without any traces of 

 colour; but the sky adjoining one was blue, adjoining the other 

 yellow. As the instrument is of considerable delicacy, we must 

 conclude that little or no polarized light is emitted by the co- 

 rona. The sky adjoining it, however, is polarized in a plane in- 

 dependent of the position of the sun, since its colour (as seen in 

 the polariscope) is the same whether above, below, or on one 

 side of it. The most probable explanation of this curious phe- 

 nomenon is, that the earth beyond 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 observation and those previously made, I have endeavoured 

 to learn what form of polariscope has heretofore been used ; but, 

 unfortunately, in most cases no description has been published. 

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

 its principal plane vertical, found strong traces of polarization in 

 this plane. This observation, however, agrees with mine if we 

 suppose that the polarization of the sky was taken for that of 

 the corona, a natural mistake with this form of instrument. 

 Another observer, who 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 verified my results with a simple prism 

 of Iceland-spar, with which two images of the corona were seen 

 precisely alike and showing no signs of polarization. \Ye can- 

 not infer from this that the corona is self-luminous, since polar- 



* Communicated by the Author. 

 Phil Mag. S. 4. Vol. 38. No. 255. Oct. 1869. U 



