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304 THE ADVANCE OF PHOTOGRAPHY
only be seen through a magnifying glass In the case of a long exposure their size depends on the more or less strong vibrations of the atmosphere, which occasion the nickering of the stars. Stars of the ninth magnitude could be photographed with an exposure of eight minutes with wet plates ; the light of these stars is ten times weaker than that of the faintest which can be detected on a clear night by the naked eye, and their images are very small points. It would be difficult to distinguish these small points from dirt spots on the plate. To do this, Rutherford made use of an ingenious device. He brought the telescope, after the first exposure of eight minutes, into a slightly different direction, and made a second exposure of eight minutes, while the clockwork continued to operate, and move the telescope correctly in this second direction.
In this manner two closely adjacent images are obtained of every star on the plate, the distance and relative position being in all the same. These double images can be easily found on the plate and distinguished from accidental spots. If the telescope stops, it is evident that the images of the stars make a movement on the plate, the bright stars describing a line. This line is of great importance to determine the direction from east to west on the plate. For faint stars which leave no line a third exposure is necessary to determine this direction. This is done after the clockwork of the telescope has been stopped for some minutes.
Rutherford took numerous views of the stars, and they will serve as important means of comparison, after the lapse of centuries, in order to discover what change has taken place in the position of the fixed stars.1
In the last few years photography has proved a very useful servant to the astronomer, in the preparation of
1 Details respecting Rutherford's observatory are contained in the " Photographischen Mittheilungen," Jahrg. 1870. Berlin : Oppenheim.