Total Solar Eclipse 0/1900 (May 28). 



363 



Pointer. 



Nicoi. 



m 



very kindly suggested that the parts of this apparatus should be put 

 into one of the compartments of the " double tube " alongside of the 

 other polariscopic apparatus which he had himself arranged. I fell in 

 with this suggestion very gladly, and the parts were taken to Algiers 

 to be fitted there. It required very careful arrangement to get the 

 two lots of apparatus into the tube, but in the end it was successfully 

 accomplished, and Professor Turner made the exposures for the Savart 

 camera simultaneously with those for his own polariscopic and other 

 cameras. The pictures obtained with the Savart camera are on the 

 same plates with the pictures obtained with Professor Turner's double 

 image polariscopic camera. 



The general procedure with the Savart camera was to be as 

 follows : The Savart and Nicol were to be rotated until the bands 

 due to the plane polarisation of the sky in front of the corona were 

 extinguished, and photographs of the corona were to be taken. But 

 it was not possible to look through the camera itself in order to make 

 the adjustment " to extinction," for this would have interrupted the 

 exposures for all the other instruments in Professor Turner's charge. 

 Accordingly a subsidiary Savart polariscope was provided, which I 

 may call the visual Savart to distinguish it from the camera Savart. 

 The visual Savart was set up in my hut, with pointer and graduated 

 circle attached, and the zero and numbering of the scale were adjusted 

 so that the readings corresponded with those of the camera Savart, 

 account being taken of the fact that the sky was seen in the camera 

 by reflection from the coalostat. 



The programme of exposures was as follows : 



1 second, 5 seconds, 20 seconds, 5 seconds, 1 second. 



The first two were made with an arbitrary setting of the Savart, 

 and the setting chosen was approximately that which would correspond 

 to extinction of bands due to vertical polarisation. Meanwhile I had 

 determined the plane of polarisation of the sky in front of the corona 

 by observations with the visual Savart, made immediately after the 

 exposures with the four-prism spectroscope were so far completed that 

 the long exposure for the corona spectrum was begun, viz., 6 seconds 



