SOME REFLECTIONS SUGGESTED B^' THE APPLTCA- 

 TION OF PHOTOGRAPHY TO ASTRONOMICAL RE- 

 SEARCH." 



By II. II. TuRNKK. I). Sc, F. R. S. 



It is a fainiliai- fact that there arc epochs in the history of a 

 science when it ac(iiiires new vio-oi-; when new branches are pnt forth 

 and ohl branches bnd afresh or blossom more plenteonsly. The vivi- 

 fying cause is generally to be found either in the majestic form of the 

 discovery of a new law of nature or- in the humbler guise of the 

 invention of a new instrument of research. The history of astron- 

 oni}^ has been rich in such epochs, notable among them being that 

 when Newton announced to the world the great law of gravitation, 

 and that when Galileo first turned his telescope to the skies. 



We have within the last half century been fortunate enough to 

 include another' great epoch in astronomical history, characterized 

 b}' the bii'th, almost a twin bii'th. of two new scientific Aveapons— the 

 spectroscopic and the sensitive film. It is, of course, somewhat diffi- 

 cult and scarcely necessary to assign an exact date for the origin of 

 either of these. The spectroscojjc was ])erhaps first systematically 

 used on the heavenly bodies by Pluggins, Rutherfurd, and Secchi in 

 the fifties, but we may trace it back to the early work of Fraunhofer, 

 who described the spectrum of Sirius in 1817, or further back to th.c 

 experiments of Newton with a i)i'ism ; and the dry plate, which in 

 jjarticular has conferred such benefits on our science, had of course 

 its pi-ecursoi's in the collodion plate or the daguei'reotype. But the 

 greater ])art of the influence on astronouiy of both the spectroscojje 

 and the photographic method dates fioui the time when the dry plate 

 was first used successfully, not much more than a (pnirter of a cen- 

 tury ago; and in that quarter of a century there have been compressed 

 new advances in our knowledge which perhaps will comj^are favor- 



« Address delivered by II. II. Turner, D. Se., F. R. S., Savilian professor of 

 astronomy in tlie University of Oxford, in the section of astrojjLysics at tlie 

 •Coiisress of Arts and Sciences at St. Louis, on Wednesday, September 21, 1!)04. 

 Reprinted from tlie Obesra\ tor.\ , Ijondoii, November-December, 1904. 



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