172 PHOTOGKAPHY IJST ASTRONOMICAL RESEARCH. 



ably 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 encour- 

 aging 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 photography, 

 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 nnich that has been said 

 already, but I hope it A^ill be clear that no claim to originality is 

 advanced. In what follows I wish to claim notliing as mine save 

 its im}?ei'fections. 



The advantages of the photographic method, which attracted atten- 

 tion from the first, may be groui)ed under three heads — its power, its 

 facility, and its accuracy. The lines of demarcation are ill defined, 

 but the classification w^ill help us a little, and I proceed to consider 

 the groups in this order. 



The immense power of the photographic method as compared 

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

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

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

 exposure. No property of the photographic plate has excited more 

 marvel than the former — that it can detect objects too faint to be 

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

 we were in ignorance and should have remained in ignorance. Early 

 successes have been followed up by others more striking as years 

 have rolled on, as l)etter instruments have been devised, and the 

 patience 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 j)ossil)ilities with the former were limited 

 to the period during which it would renuiin Avet; with the latter, 

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

 course, continuously in the case of astronomical ])hot()graphy, for 

 the camera must be closed when daylight api)roaches; but it can 

 be opened again at nightfall aud the exposure resumed without 

 fault. In this way objects of extraordinary faintness have been 

 revealed to us. AA'hen Nova I'ersei had flashetl into brillianct^ in 

 1901, and then slowly I'adtMl, long-ex})osure j)hotographs of its region 

 ivvealed to us a faint nebulous structure which we could never 

 have seen; they told us that this structure was changing in aj^pear- 

 ance in a manner which it taxed oui- ingejuiity to explain, and about 

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



