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ASTRONOMICAL PHOTOGRAPHY 317
brightness, stars of the first magnitude being 25 times as bright as stars of the second, those of the second 2*5 times as bright as those of the third, and so on ; hence a star of the first magnitude is about 100 times as bright as a star of the fifth magnitude, and 250 times as bright as one of the sixth, which is the faintest star ever visible to the naked eye.
This little satellite, being only of the seventeenth magnitude, can easily be seen to be about ^^ the brightness of these just visible stars, thus it must have required not only considerable skill, but a great deal of patience in deciphering the photographic records which contain the first evidence of its existence, since, although it changed its position among the neighbouring stars, the very faint impression was so indistinct as to make it quite probable that it was due to some flaw in the film. However.at the end of February 1908, Melotte was certain of the existence of the satellite, and it was then found that it could be traced back to January 27th on the photographic plates.
The orbit of the satellite was then calculated by Messrs Crommelin and Cowell from the records which had been obtained, and so soon as Jupiter was again in a favourable position (January 1909), and the sky was dark, and the night bright, Mr Melotte succeeded in again obtaining a photograph of the eighth satellite.
Its position was found to be very close to that indicated by the above-named astronomers, and it has been thus proved that this very small satellite of Jupiter is a very distant one, with retrograde motion and a very eccentric orbit. This brilliant success in astronomical research is one of the latest triumphs in that branch of science due to the help of photography in recording the positions and motions of the heavenly bodies.
Effect of a Brief Exposure of Plates to Light when they are used for photographing Faint Spectra. — R. W. Wood has