PHOTOGRAPHS OF THE CORONA. 103 



of the corona which they depict, 1871 being as remarked in 

 the preceding chapter just after the sun-spot maximum, and 

 1878 being almost dead minimum. The wonderful complexity 

 of structure, shown by the coronal photographs in the earlier 

 year, was replaced accordingly by an almost severe simplicity in 

 the later. 



This eclipse of 1878 offered a new problem in coronal 

 photography. Two of the observers of that eclipse Professor 

 Newcomb in Wyoming, and Professor Langley on Pike's Peak, 

 Colorado observed the great equatorial wings of the corona 

 extending eleven millions of miles east and west of the sun, 

 whilst the photographs do not show them further than one 

 million. It became a matter of some interest, therefore, to see 

 if it would not be possible to photograph these extensions ; and 

 Father Perry, in the eclipse expedition of 1889, December 22 

 (which cost him his life), included the attempt to photograph 

 these streamers in his programme. 



In the eclipses from 1870 to 1878 the old wet-plate collodion 

 process was used. In more recent years, gelatine dry-plates 

 have almost entirely superseded them. From 1882 and onwards 

 the official expeditions sent out by the British Government 

 have in every case been provided with one or two photographic 

 telescopes of 4 inches aperture and about 5 feet focal length, 

 and a valuable and homogeneous series of negatives have been 

 accumulated by their means. In 1893 Professor Schaeberle, 

 from the Lick Observatory, introduced in his station in Chili 

 the use of a lens of very long focus, giving an image of the 

 sun over 4 inches in diameter, and obviating the difficulty of 

 mounting a telescope 40 feet in length equatorially by making 

 it a fixture, and simply moving the plate. In the same eclipse 

 the British official expeditions were supplied with "double 

 cameras." These " double cameras " each carried a pair of 

 4-inch photographic lenses, and the plates were exposed in the 

 primary focus of one of these lenses so as to carry on the series 

 begun in 1882, whilst the other lens was fitted with a Dallmeyer 

 telephoto lens, so as to give an image of the corona on a scale 

 about two and a half times as great as the companion negative 

 in the primary focus. The latter principle was also adopted by 

 the Astronomer Royal in the Indian eclipse of 1898 to give a 

 picture on the 4-inch scale with an object glass of 9 inches 

 aperture and nearly 9 feet focal length. 



The eclipse of 1896, August 9, yielded a successful attempt 

 to photograph the coronal rays at Novaya Zemlya, where 

 M. Hansky, of the Russian expedition, secured a coronal streamer 

 some two solar diameters in length with a lens of 2 inches 

 aperture and 12| inches focus. 



1896 saw another modification in eclipse instruments, namely 

 the use of telescopes rigidly mounted and fed by a " coelostat," 



