ADDRESS. 31 



type, while the bright star is solar in character, the composite spectrum 

 would be solar with the hydrogen lines unusually strong. Such a spec- 

 trum would in itself afford some probability of a double origin, and 

 suggest the existence of a companion star. 



In the case of a true binary star the orbital motions of the pair would 

 reveal themselves in a small periodical swaying of the hydrogen lines 

 relatively to the solar ones. 



Professor Pickering considers that his photographs show ten stars 

 with composite spectra ; of these, five are known to be double. The 

 others are : t Persei, I AurigiB, 8 Sagittarii, 31 Ceti, and /3 Capricorni. 

 Perhaps ^ Ljrae should be added to this list. 



In his recent classical work on the rotation of the sun, Duner has 

 not only determined the solar rotation for the equator but for different 

 parallels of latitude up to 75°. The close accordance of his results shows 

 that these observations are suflBciently accurate to be discussed with the 

 variation of the solar rotation for different latitudes, which had been 

 determined by the older astronomical methods from the observations of 

 the solar spots. 



Though I have already spoken incidentally of the invaluable aid 

 which is furnished by photography in some of the applications of the 

 spectroscope to the heavenly bodies, the new power which modern 

 photography has put into the hands of the astronomer is so great, and 

 has led already, within the last few years, to new acquisitions of know- 

 ledge of such vast importance, that it is fitting that a few sentences 

 should be specially devoted to this subject. 



Photography is no new discovery, being about half a century old ; 

 it may excite surprise, and indeed possibly suggest some apathy on the 

 part of astronomers, that though the suggestion of the application of 

 photography to the heavenly bodies dates from the memorable occasion 

 when, in 1839, Arago, announcing to the Academie des Sciences the great 

 discovery of Niepce and Daguerre, spoke of the possibility of taking 

 pictures of the sun and moon by the new process, jet that it is only 

 within a few years that notable advances in astronomical methods and 

 discovery have been made by its aid. 



The explanation is to be found in the comparative unsuitability of 

 the earlier photographic methods for use in the observatory. Injustice 

 to the early workers in astronomical photography, among whom Bond, 

 De la Rue, J. W. Draper, Rutherfurd, Gould, hold a foremost place, it is 

 needful to state clearly that the recent great successes in astronomical 

 photography are not due to greater skill, nor, to any great extent, to 

 superior instruments, but to the very great advantages which the modern 

 gelatine dry plate possesses for use in the observatory over the methods 

 of Dnguerre, and even over the wet collodion film on glass which, though 

 a fr eat advance on the silver plate, went but a little way towards putting- 



