Page:Popular Science Monthly Volume 22.djvu/503

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METHODS IN MODERN PHYSICAL ASTRONOMY.

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 photography 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 nebulæ. Mr. Draper, in America, and the observatory at Meudon, have obtained photographs of the nebulæ in Orion. The nebulæ 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. Consequently, we are liable to have images of the same nebulæ, in no way comparable 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 fugacious, 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 perfected 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 thousandth 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 nebulæ 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 obscurity 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 opposite regions of dark heat in which the eye has no power.

The priceless advantages which photography offers for the prosecution of our experiments are, in short, the preservation of the images, the extension of sensibility, and the faculty of seizing phenomena of the most different degrees of illumination, including the extremely strong and the extremely weak.

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