432 ASTROPHYSICS 



ity is advanced; in what follows I wish to claim nothing as mine 

 save its imperfections. 



The advantages of the photographic method, which attracted 

 attention from the first, may be grouped under three heads its 

 power, its facility, and its accuracy. The lines of demarcation are 

 ill-defined, but the classification will 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 e}^e arises from the two facts that (a) by the accumulation 

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

 that (&) 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 better 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 possibilities with the former were limited 

 to the period during which it would remain wet; with the latter, 

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

 course, continually 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. When 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 which we could never have seen; 

 they told us that this structure was changing in appearance in a 

 manner which it taxed our ingenuity to explain, and about which 

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

 the spectrum of this faint object has been photographed. When we 

 consider that in the spectrum each point of light in the object is 

 enormously diluted by being spread out into a line, the difficulty 

 of this undertaking seemed almost prohibitive; but it was not 

 sufficient to prevent Mr. Perrine, of the Lick Observatory, from 

 making the attempt, and he was deservedly rewarded by success. 1 

 may be wrong in regarding this success as the high-water mark in 

 this direction at the present time, and it will probably be surpassed 

 by some new achievement very shortly ; but it will serve to illustrate 

 the power of photography in dealing with faint objects. 



But may we here pause for one moment to marvel at the sensi- 

 tiveness of the human eye, which is such that it is, after all, not left 

 very far behind in the race? The eye, sensitive as it is merely to 



