664i ' REPORT— 1900. 



from tlie other source of light, according as one is moving towards or away from 

 the earth. This displacement of the spectrum lines led to the discovery of the 

 duplicity of /3 Aurigae, and ( Urssc Majoris by Pickering.' 



Several other such stars have now been detected, notably /3 Lyrse, and 

 lastly Capella, discovered independently by Campbell - at Lick, and Newall ^ at 

 Cambridge. 



The progress of the new astronomy is so closely bound up with that of photo- 

 graphy that I shall briefly call to mind some of the many achievements in which 

 photography has aided the astronomer. 



Daguerre's invention in 1839 was almost immediately tried with the sun and 

 ' moon, J. W. Draper and the two Bonds in America, Warren de la Rue in this 

 country, and Foucault and Fizeau in France, being among the pioneers of celestial 

 photography ; but no real progress seems to have been made until after the intro- 

 duction of the collodion process. Sir John Herschel in 1847 suggested the daily 

 self-registration of the sun-spots to supersede drawings ; and in 1857 the De la Rue 

 photo-heliograph was installed at Kew. From 1858-72 a daily record was main- 

 tained by the Kew photo-heliograph, when the work was discontinued. Since 

 1873 the Kew series has been continued at Greenwich, which is supplemented by 

 pictures from Dehra Diin in India and from Mauritius. The standard size of the 

 sun's disc on these photographs has now been for many years 8 inches, though for 

 some time a 12-inch series was kept up. 



The first recorded endeavour to employ photography for eclipse work dates back 

 to 1851, when Berowsky obtained a daguerreotype of the solar prominences during 

 the total eclipse. From that date nearly every . total eclipse of the sun has been 

 studied by the aid of photography. 



In 1860 the first regularly planned attack on the problem by means of photo- 

 graphy was made, when De la Rue and Secchi successfully photogi'aphed the pro- 

 minences and traces of the corona, but it was not until 1869 that Professor Stephen 

 Alexander obtained the first good photograph of the corona. 



In recent years, from 1893 up to the total eclipse which occurred last May, 

 photography has been employed to secure large-scale pictures of the corona. 

 These were inaugurated in 1893 by Professor Schaeberle, who secured a 4-inch 

 picture of the eclipsed sun in Chili: these have been exceeded by Professor Langley, 

 who obtained a 15-inch picture of the corona in North Carolina during the eclipse 

 of May 1900. 



Photography also supplied the key to the question of the prominences and 

 corona being solar appendages, for pictures of the eclipse sun taken in Spain in 1860 

 terminated this dispute with regard to the prominences, and finally to the corona in 



In 1876, in addition to photographing the corona, attempts were made to 

 photograph its spectrum, and at every eclipse since then the sensitised plate has 

 been used to record both the spectrum of the chromosphere and the corona. The 

 spectrum of the lower layers of the chromosphere was first successfully photo- 

 graphed during the total eclipse of 1896 in Nova ZemblabyMr. Shackletoii, though 

 seen by Young as early as 1870, and a new value was given to the wave-length of 

 the coronal line (wrongly mapped by Young in 1 869) from photographs taken by 

 Mr. Fowler during the eclipse of 1898 (India). 



Lunar pliotogTaphy has occupied the attention of various physicists from time 

 to time, and when Daguerre's process was first enunciated, Arago proposed that 

 the lunar surface should be studied by means of the photographically produced 

 images. In 1840 Dr. Draper succeeded in impressing a daguerreotype plate with a 

 lunar image by the aid of a 5-inch refractor. The earliest lunar photographs, how- 

 ever, shown in England were due to Professor Bond, of the United States. These 

 he exhibited at the Great Exhibition in 1851. Dancer, the optician, of Manchester, 

 was perhaps the first Englishman who secured lunar images, but they were of 

 small size.* 



Another skilful observer was Crookes, who obtained images of 2 inches 



' Am. Jour. (.3), 39, p. 46 (1890). - Astro-Phys. Jour., vol. s. p. 177. 



' MmitJily Notices, vol. Ix, p. 2 (1899). ■• Abney, Fhotography, 



