242 Mr. H. F. Netvall [Feb. 9, 



shall get an estimate of the relative quantity of polarisation in 

 the corona. 



I have been fortunate enough to get three series of photographs 

 of this kind, two of them taken in 1900 and 1901 at the minimum of 

 sun spots, and the third in 1905 at the maximum. The comparison 

 of these will show whether there is a variation in the amount of light 

 due to polarisation in the corona. 



Professor Turner has also carried out experiments of a similar 

 nature with different apparatus and on new lines at Assuan. I wish 

 that I had the material to show you which he gathered in the ecUpse ; 

 his researches seem to show that something between one-third of the 

 Hght and half of the light is polarised, and therefore presumably 

 reflected from dust in the corona. 



Then I come to another set of our observations at Guelma. With 

 Mr. Cooke's assistance we managed to get four photographs of the 

 corona with an analysing polariscope (a large Nicol prism, which 

 transmitted a two-inch beam) in four different positions — vertical, 

 45° to the east, 45° to the west, and horizontal. Here I show you 

 the photograph taken with the Nicol prism in the vertical position ; 

 it shows the vertical component of the light of the corona. The 

 Nicol prism was placed parallel to this rod, so that no light which 

 was polarised in a plane perpendicular to this rod would leave its 

 effect upon this plate. 



The next photograph shows the result obtained by putting the 

 Nicol with its principal plane at an inclination of 45° to the east. 



The next one shows us the picture obtained when the Nicol was 

 incHned at 45° to the west. The pictures all exhibit the greatest 

 extensions of corona in directions parallel to the long diagonal of the 

 Nicol. The polarisation of the corona is seen at a glance to be radial. 

 In a composite slide which I will show you presently, we shall have 

 means of comparing the polarised pictures simultaneously, for we shall 

 see them side by side. 



But, before doing that, I will show you the fourth photograph of 

 the series ; for it will give you an idea what an eclipse-failure looks 

 like. Each of the four photographs was exposed for 80 seconds ; 

 but, unfortunately, during the last few seconds of the exposure of 

 the fourth picture, the sun had reappeared from behind the moon. 

 One sees the corona just indicated round here, and also the black 

 body of the moon. And here is the edge of the brilliant crescent- 

 sun protruding from behind the eclipsing moon. It is so bright that 

 it is reversed, and looks black, except at its edges. 



I have called your attention in another photograph to certain 

 bright, curved arches, frequently seen in the corona, as if centred 

 round a prominence, and I have raised the question whether the 

 luminosity of these rings is produced by the bombardment of cor- 

 puscles. If they are, somehow, the result of incandescence in the 

 gas, they would not show polarisation. Do these arches behave 



