Total Solar Eclipse of 1900 (May 28). 365 



so that the atmospheric bands may be in opposite phase i.e., with 

 black central band to the coronal bands. 



By a very fortunate accident just such an imperfection has arisen in 

 the case of the plate No. 3, for the bright bands on the corona fall on 

 dark atmospheric bands. It might be that the curvature of Savart's 

 bands, which theoretically exists, misleads one ; but a tolerably careful 

 examination of the faint bands shows them to be sensibly straight in 

 the limited field dealt with, and the antagonism of the bands leaves no 

 possible doubt that the bands seen on the corona are due to the polari- 

 sation of the corona. 



It is difficult to reconcile the marked polarisation evidenced in this 

 investigation with the absence of Fraunhofer lines in the spectrum of 

 the corona. 



Across the dark moon no atmospheric bands are discernible, and 

 there appears to be no doubt that photographically the dark moon is 

 darker than the sky. These are points that need explanation. An 

 investigation of the real facts would be difficult, but none the less 

 interesting ; for the idea suggested by much of the evidence along 

 different lines is that some of the light which is usually attributed to 

 the sky may come from beyond the moon. For instance, is a milky 

 sky on a moonless night simply the result of starlight scattered by 

 the processes producing scintillation, or are other causes at work 1 



27. Atmospheric Polarisation. 



Preparations had been made that a systematic survey of the polarisa- 

 tion of the sky should be undertaken during the eclipse, with a view 

 to determining the plane of polarisation in various quarters of the sky, 

 a more precise knowledge of the general distribution of polarisation 

 being needed for the explanation of some of the anomalies that appear 

 to have been observed with respect to the atmospheric effects in pre- 

 vious eclipses. 



Nine Savart polariscopes were mounted in similar turning tubes, 

 provided with pointers and graduated circles, and attached to wooden 

 stands. The stands were arranged so that each carried two polari- 

 scopes ; one pointed to the horizon, the other to a point 30 above the 

 horizon. The four stands were fixed on the top of a tall box on the 

 balcony of the equatorial coude which M. Trepied had kindly put at 

 our disposal. The polariscopes were directed towards the four 

 quarters of the sky, N.E., N.W., S.W., and S.E. During the eclipse 

 the sun was at an altitude of 30, and only a few degrees north of 

 west ; thus the polariscopes were directed to points nearly symmetri- 

 cally disposed with regard to the sun. All the Savart plates were 

 fixed relatively to the Nicol prisms, so that the bands were parallel 

 to the plane of polarisation of the light transmitted by the Nicols. 



