Page:Proceedings of the Royal Society of London Vol 69.djvu/258

242 photographs, which was a somewhat trying and arduous matter, was carried out very satisfactorily, only one failure of any consequence occurring.

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

vol. 67, p. 397.
 * ' Monthly Notices, K.A..S.,' vol.57, p. 105; 'Eoy. Soc. Proc.,' vol. 64, p. 8;