PHOTOGRAPHY AND ASTROPHYSICS 431 



anthropologist cannot wait; with him it is now or never, and science 

 would be a poor thing indeed if we could not be so unselfish as to 

 recognize his needs as more urgent than our own. Is it too much to 

 hope that, even before we leave this hospitable city, we may have 

 some assurance that full justice shall be done in this matter? 



It is a familiar fact that there are epochs in the history of a science 

 when it acquires new vigor; when new branches are put forth and 

 old branches bud afresh or blossom more plenteously. The vivifying 

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

 covery of a new law of nature, or in the humbler guise of the inven- 

 tion of a new instrument of research. The history of astronomy 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 

 by the birth, almost a twin-birth, of two new scientific weapons 

 the spectroscope and the sensitive film. It is, of course, somewhat 

 difficult and scarcely necessary to assign an exact date for the origin 

 of either of these; the spectroscope was perhaps first systematically 

 used on the heavenly bodies by Huggins, 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 the 

 experiments of Newton with a prism; and the dry plate, which in 

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

 its precursors in the collodion plate or the daguerreotype. But the 

 greater part of the influence on astronomy of both the spectroscope 

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

 was first used successfully, not much more than a quarter of a century 

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

 new advances in our knowledge which perhaps will compare favorably 

 with the work of any similar period in centuries either past or to come. 

 It is difficult to estimate at their true value historical events in which 

 we play a part, and any review of such a period undertaken now 

 must be necessarily imperfect, for we are advancing so rapidly that 

 our point of view is continually changing. But it is an encouraging 

 thought that obvious difficulties may enhance interest in the attempt 

 and suggest kindly excuses for its shortcomings. 



From the embarrassingly large number of possible topics which 

 the period provides, I have selected that of astronomical photo- 

 graphy, and I invite your attention to some characteristic features 

 of the photographic method in astronomy, and some reflections 

 thereupon. It is scarcely possible to avoid repeating much that has 

 been said already, but I hope it will be clear that no claim to original- 



