1906.] on Eclipse ProhJems and Observations. 241 



prominences. Here is a detached prominence, far above the sun's 

 surface. 



But let me call jour special attention to the alternating bright 

 and dark arches round a single prominence. They are features in all 

 coronas. They are difficult to explain, but this is perhaps an ocaision 

 for seeing if any of our seeds will grow. 



Where these arches are formed, that is, at a considerable distance 

 from the surface of the sun, the atmosphere of the sun would be 

 quite inappreciable ; it would be far more rarefied than the earth's 

 atmosphere one hundred miles above the surface of the earth. Yet 

 our splinters assure us that there is matter in the close neighbourhood 

 of the sun. Under the scorching rays vapour must be given off, as 

 rarefied as any comet's tail. Is it a flood or flight of corpuscles, 

 coming from an incandescent prominence and passing through this 

 matter, that gives this luminosity in the form of an arch, which is the 

 perspective appearance of some expanding surface spreading outwards 

 from the prominence, because it takes about the same time for the 

 particles to travel outwards from the prominences ? Such an explana- 

 tion is not without difficulties. I will revert to it again later. 



Meanwhile I show you other eclipse records dealing with work 

 which was carried out at Guelma by our party, and in which I had 

 the good fortune to be assisted by Dr. Wallace, and by Mr. Champion, 

 and Mr. Cooke, and Mr. Wadmore. The first set deals with studies 

 of the polarisation of the corona, with a view of finding out what 

 proportion of the light of the corona is polarised and hence due to 

 light reflected by the dust particles in the neighbourhood of the sun, 

 or possibly to some other unrecognised source of polarisation. These 

 photographs were taken with a camera in front of which certain 

 crystalline plates are put together with a Nicol prism as analyser, 

 forming what I have called a Savart camera. AVhen the light of the 

 blue sky falls on the camera, the polarisation of the light is shown by 

 this banded appearance. When the corona is photographed by means 

 of such a camera, then if the Hght is polarised, bands should appear 

 over the pictures of the corona taken by the camera. The distribu- 

 tion of the bands in the records obtained at Guelma is somewhat 

 complicated, but it shows that the light is strongly polarised, and 

 that the polarisation is radial. 



This research is simply a special continuation of work which has 

 been done in other ways by several eclipse observers of recent years, 

 and serves to give exact determinations of the relative quantities of 

 polarised and unpolarised light in the corona. The result which we 

 get in photographing the corona shows no doubt about the existence 

 and the nature of the polarisation. I show you another picture where 

 we have two photographs side by side, one taken with an ordinary 

 camera and one with the Savart camera. It will be evident 

 that by measurement and comparison of the alternations of bright 

 and faint light along a given line on the two photographs, we 

 YoL. XYIIL (Xo. 100) tt 



