Report on the Solar Eclipse Expedition to Sahdol. 



PART II. SEPARATE REPORT BY MR. W. H. M. CHRISTIE. 



The programme of observation was composed of two distinct parts 

 (1) Photographs of the corona on a large scale during totality; (2) 

 photographs of the partial phase before and after totality for 

 determination of the position of the moon relatively to the sun. 



The instrument used in both cases was the photographic telescope 

 by Grubb, with object-glass of 9 inches aperture and 8 ft. 6 in. focal 

 length (presented to the Royal Observatory by Sir Henry Thompson), 

 to which a concave compound lens by Dallmeyer, of 3 inches diameter 

 and 12 inches focus, had been fitted as a secondary magnifier, placed 

 a short distance within the focus. This combination gave an image 

 of the sun 4 inches in diameter, and a field (for full pencils) of nearly 

 10 inches diameter, so that the corona to a distance of one and a half 

 radii from the Sun's limb would be included in the field. A ccelostat, 

 specially designed by Dr. Common, carrying a plane silver-on-glass 

 mirror of 16 inches diameter, made by him, was employed to reflect 

 the rays into the Thompson coronagraph, which was firmly mounted 

 on two brick piers, so as to point to the mirror at an angle of 

 depression of about 10, and to be placed in an azimuth of about 

 17 north of west for the day of the eclipse. The camera was 

 furnished with eight plateholders, taking 12 x 10 in. plates, seven 

 being reserved for use during totality, and the eighth fitted with a 

 Thornton-Pickard instantaneous focal-plane shutter, to take photo- 

 graphs on 8 J x 6^ in. plates during the partial phases, for determination, 

 of the moon's position. 



The seven slides for photographs of the corona during totality 

 were exposed as below, the exposures being given with a screen held 



