The advance of photography : its history and modern applications (1911)

282 THE ADVANCE OF PHOTOGRAPHY will be seen b}T a perusal of the following very brief account of the use of photography in this branch of science. Why Photography is useful in Astronomical Work. — There are several very important reasons why the use of photography has become so general in astronomical studies. In the first place, the sensitive plate will accomplish in an extremely short time that for which an artist would require many hours, and the astronomical objects are often such as very rapidly alter in detail. Hence this seems the only reasonable way of obtaining a series of views showing the changing appearances of the heavenly body concerned. Then again, although, even after all the necessary precautions are taken, the photographic plate will not be absolutely panchromatic, yet it will be free from bias and will not suffer in any way from optical illusions such as only, too often, affect the work of the observer ; while lastly, it may fairly be said that it would be next to impossible to represent by any other means the minute details and delicate shades of difference in nebula? and comets. But perhaps one of the most important applications is due to the fact that the action of light upon the plate has a cumulative effect. When a person looks through a telescope, all the stars which can be seen by its aid become visible at once ; no amount of careful, continued watching will help the observer to see more stars ; on the other hand, the eyes may tire and so even less be seen. Now, with a sensitive plate, just the reverse is true. Each beam of light received adds its portion to the general effect, and so although the light received from a faint star may not visibly affect the plate if only exposed for one minute, quite a different result may be obtained if the exposure is doubled. Thus it is that by the aid of photography many millions