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The glimpses of the corona that I was fortunate enough to get in 

 India in 1898 through a small Savart polariscope convince' I me that 

 that instrument, if properly used, would give just the information that 

 is wanted to decide some of the perplexing points that still survive in 

 the spectroscopic and polariscopic study of the corona. The chief 

 objection is that the phenomena are far too complicated to study by 

 eye observations in the short time at one's disposal in an eclipse. 

 Here is a case in which photographic methods should certainly U> 

 adopted if possible. Shortly after my return from India in 1898 I 

 made some experiments to test the feasibility of photographing 

 Savart's bands, and met with such promising success that no doubt 

 was left in my mind that a photographic record of the distribution of 

 Savart bands over the corona would give good results in supplement- 

 ing the work which Professor Turner has in recent eclipses 1>een 

 carrying out in studying the polariscopic phenomena of the corona. 

 Accordingly when I was asked to take part in the observations of the 

 eclipse at Algiers, it seemed well to make arrangements to prepare a 

 polariscopic camera. 



Professor J. J. Thomson very kindly put two large Nicol prisms at 

 my disposal. The aperture of these prisms is 1 inches. It was thus 

 possible to use a lens of a focal length of about 3 feet, if suitable 

 Savart plates could be found. On making the necessary calculations 

 I found that the plates would have to be 15 mm. thick, cut in quartz 

 at 45* to the axis of the crystal. A pair of such plates would give 

 Iwinds of the desired closeness, viz., alxjut 10' apart, instead of the 

 usual 1 or 1 30'. Fortunately Mr. Hilger was able to cut a slab, 

 from the sloping top of a quartz crystal that I had in my possession, 

 large enough to make two plates, each 14 mm. thick, of circular 

 section, and with a diameter of 39 mm. (1 inches). The whole slab 

 was worked and polished with plane parallel surfaces, so as to secure 

 equality of thickness, and was then cut into two parts, which were 

 combined in the usual manner. 



The figure shows diagrammatically the arrangement of the camera 

 with the Savart plates and Nicol prism in front. The lens was a 

 3^-inch lens of focal length 40 inches. The aperture was reduced to 

 \\ inches, or approximately F/27 for central pencils. The Savart 

 plates were fixed to the Nicol prism so that the bands were parallel to 

 the plane of polarisation of the light transmitted by the Nicol. The 

 whole system was arranged so that it could be rotated on its axis into 

 any desired position, and a pointer was provided so that the position 

 could be read off a large circle. 



In discussing with Professor Turner the arrangements for the 

 various items in the programme of observations to be carried out, he 



