Report on the Solar Eclipse Expedition to Salidol. 



11 



diameter of the sun's image thrown on the slit being 0'21 inch, it was 

 thought best to take the two photographs, so as to include opposite 

 limbs of the eclipsed sun, and as much of the corona as could be 

 got. The change from one portion of the sun to another was 

 arranged by having two slits as follows : 



The two slits were cut in a piece of blackened card, the actual 

 size of which is shown in the diagram. If the line ABCD had 

 been diametral to the sun's or moon's image, half the moon and 

 corona would in the first instance have appeared in the middle of the 

 lower slit, CD. In this position the first exposure was given, pro- 

 ducing on the plate two images side by side of the right limb of the 

 eclipsed sun, one image being due to light polarised in a plane 



parallel to CD, and the other to light polarised in the perpendicular 

 plane. For the next exposure the card was slipped down in the 

 groove in which it fitted lightly, and the left limb of the eclipsed sun 

 fell on the middle of the slit AB ; and so two images of the rest of 

 the corona would be obtained. By combining the two photographs, 

 we could get a double picture of a slice of the corona 1*0 inch long 

 and 0*40 inch wide, containing the moon centrally ; or, translating 

 inches into diameters of the moon's image, five diameters long and 

 two diameters wide. As a matter of fact, the image of the sun was 

 not adjusted centrally on the line ABCD, and a good deal more of 

 the moon's disc appeared on one photograph than on the other, this 

 being deliberately arranged for a special reason, which will appear 

 in discussing the photographs obtained. 



Instrumental Adjustments. 



5. Adjustment of Coslostat. The adjustment of the polar axis was 

 made as described in the Report on the Japan Expedition* by means 

 of the attached declination theodolites. 



The following are the actual observations, those with the level 



* ' Monthly Notices,' vol. 57, p. 102. 



