172 PHOTOGKAl'HY IN ASTRONOMICAL RESEARCH. 



;il)ly with the work of any simihir period in centuries either past or 

 to come. Tt is difticult to estimate at their true vahie historical events 

 in which we \)h\y a part, and any review of such a period undertaken 

 now nmst he necessarily imperfect, for we ai-e advancing so rapidly 

 that our point of view is contiinially chan<>iiio-. p,iit it is an encour- 

 aging thought that obvious ditliculties may enhance interest in the 

 attempt and suggest kindly excuses for its shoitcomings. 



From the embarrassingly large number of })ossible topics which the 

 period provides I have selected that of astronomical photogi-aphy, 

 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 nuich that has been said 

 already, but I hope it will be clear that no claim to originality is 

 advanced. In what follows I wish to claim nothing as mine save 

 its imperfections. 



The advantages of the photographic method, which attracted atten- 

 tion from the first, may be grouped under thi'ee heads — its power, its 

 facility, and its accuracy. The lines of denun-cation are ill defined, 

 l)ut tlie classification will help us a little, and I proc^eed to consider 

 the groups in this order. 



The innnense power of the photographic method as compared 

 with the eye arises from the two facts that {(i) by the accumulation 

 of long exposures fainter and fainter objects can be detected, and 

 that (7^) large regions of the heavens can be recorded at the same 

 exposure. No property of the i)liotographic j^late has excited more 

 marvel than the former — that it can detect oI)jects too faint to be 

 seen even by our largest telescopes; objects of whose very existence 

 we were in ignorance and should haxc i-emained in ignorance. Earh^ 

 .successes have been followed up by others more striking as years 

 have rolled on, as better instruments have been devised, and the 

 ])atience of the watchers has proved equal to greater strain. It is 

 here -that the change from the '^ wet " plate to the " dry '" has proved 

 most advantageous. The possibilities with the former were limited 

 to the period chiring which it would icuiain wet; with the latter, 

 exposures may be continued for hours, days, even years — not, of 

 course, continuously in the case of astronomical photography, for 

 the camera must be closed when daylight approaches; but it can 

 be opened again at nightfall and the exposure resumed without 

 fault. In this way objects of extraordinary faintness have been 

 revealed to us. AVhen Nova Persei had flashed into brilliance in 

 1901, and then slowly faded, long-exposure photographs of its region 

 revealed to us a faint nebulous structure Avhich we could ncA'er 

 have seen; they told us that this structure Avas changing in api)ear- 

 ance in a manner which it taxed our ingeiniity to explain, and about 

 which speculation is still rife. But a greater triumph was to come; 



