METHODS IN MODERN PHYSICAL ASTRONOMY. 487 



brightness with which it appears to shine, is, at only a few decrees 

 from the nucleus, two or three hundred times less luminous than the 

 moon. There will doubtless be room enough to perfect these first 

 efforts, for it will be of the highest importance to obtain by photogra- 

 phy incontestable documents for the history of these stars, the nature 

 of which still presents so many enigmas. 



Equally interesting efforts have been made with respect to the 

 nebula?. Mr. Draper, in America, and the observatory at Meudon, 

 have obtained photographs of the nebulas in Orion. The nebula? are 

 of great importance in their bearing on the theory of the formation 

 of stellar systems and the genesis of worlds. It would be immensely 

 interesting to establish clearly the existence and the nature of changes 

 going on in their structure, and good photographs of them would be 

 valuable for this. They are, however, difficult subjects, on account of 

 the extreme weakness of their light, the uncertainty of their outlines, 

 and the variations of brightness in their different parts. Consequent- 

 ly, we are liable to have images of the same nebula?, in no way com- 

 parable with each other, but varying according to the length of the 

 exposure, the clearness of the sky, and the sensitiveness of the plate ; 

 and it becomes imperiously necessary to define the conditions under 

 which the images are obtained. 



The images of any object impressed by light upon the eye are fu- 

 gacious, and can be of only a limited intensity. The images fixed 

 upon the photographic plate are permanent, and can be made of an 

 intensity that becomes cumulative with the duration of the exposure. 

 The photographic retina may be expected, when the art has been per- 

 fected in the highest degree, to give us images corresponding with an 

 extremely expanded range in the duration of the exposure. We now 

 obtain photographic impressions of the sun in the one hundred thou- 

 sandth part of a second, and can not yet guess what the final limit will 

 be in the direction of brevity. On the other hand, the images of the 

 comet required an hour, and that of the nebula? in Orion more than 

 three hours of luminous action. Thus the luminous action was more 

 than five hundred million times as long in the last case as in the first. 

 What phenomena can have wide enough ranges in brightness or ob- 

 scurity to escape so admirable an elasticity ? 



The photographic plates, moreover, which are prepared now, are 

 not only sensitive to all the elementary rays which excite the retina, 

 but the power also extends into those ultra-violet regions and the op- 

 posite regions of dark heat in which the eye has no power. 



The priceless advantages which photography offers for the prose- 

 cution of our experiments are, in short, the preservation of the im- 

 ages, the extension of sensibility, and the faculty of seizing phenom- 

 ena of the most different degrees of illumination, including the ex- 

 tremely strong and the extremely weak. 



The above is a very incomplete picture of what has been accom- 



