242 Mr. F. W. Dyson. 



photographs, which was a somewhat trying and arduous matter, was 

 carried out very satisfactorily, only one failure of any consequence 

 occurring. 



II. Photographs of the Corona. 



The programme of observations was composed of two distinct parts : 



(1.) Photographs of the corona on a large scale to show structural 

 detail. 



(2.) Photographs on a smaller scale with rapid lenses, to show the 

 coronal streamers with the greatest possible extension, and to photo- 

 graph the sky round the Sun for the detection of an intra-Mercurial 

 planet, should one exist. 



(1.) Large-scale Photographs. The instrument used was the Thompson 

 photographic telescope with object glass of 9 inches aperture and 

 8 feet 6 inches focal length, belonging to the Royal Observatory, in 

 combination with a concave telephoto lens by Dallmeyer, of 4 inches 

 aperture and 16 inches focus, fitted as a secondary magnifier, to give 

 an image of the sun 4 inches in diameter, with a field (for full 

 pencils) of 14 inches. The total length of the coronagraph was 

 12 feet, the equivalent focal length being about 36 feet. The focus 

 was determined by the method used by the Astronomer Royal at 

 the eclipses of 1896, 1898, and 1900, by means of the image of a 

 gauze net in the plane of the plate reflected from the plane mirror of 

 the coelostat.* In the determination of the focus, which was done at 

 night, Mr. Curtain, warrant officer, rendered great assistance. A 

 coelostat, with 16-inch plane mirror (made by Dr. Common), was 

 employed to reflect the rays into the coronagraph, which pointed 

 downwards to the mirror at an angle of 3 or 4, and was in the 

 azimuth 18 north of east on the day of the eclipse. The adjustment 

 of the ccelostat was readily made by observations of the Sun with 

 the attached theodolite in the usual manner. 



The camera was furnished with eight plate holders, to take 

 15 x 15 inch plates, or for the shorter exposures 12 x 10 inch 

 plates in a carrier. 



Two of the photographs were taken with a Burckhalter appa- 

 ratus, made at the Royal Observatory, for graduating the length 

 of exposure at different distances from the Sun's limb. For this 

 purpose a zinc screen, whose form is shown in the diagram, and 

 which was arranged to give what appeared to be suitable exposures 

 at different distances (as shown by Professor Burckhalter's previous 

 experience and Professor Turner's investigation of the diminution of 

 the intensity of the coronal light) was rotated in front of the plate. 



To accomplish this, photographic plates were obtained with a hole 



* ' Monthly Notices, K.A..S.,' vol.57, p. 105; 'Eoy. Soc. Proc.,' vol. 64, p. 8; 

 vol. 67, p. 397. 



' 



