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ASTRONOMICAL PHOTOGRAPHY 303
sixty years ago by Professor Bond, of Cambridge, Massachusetts, but it was Mr Lewis Rutherford, of New York, who did much to perfect the method. He constructed a photographic objective of 11 inches diameter and about 13 feet focus. This objective shows a considerable focal difference ; that is, the violet and blue rays have a different focus from the yellow and red. If a clear image of the star is focussed, the sensitive plate would be adjusted to the focus of the yellow rays, and the focus of the chemically operative blue rays would fall beyond the sensitive plate, and an indistinct picture would result. The plate must therefore be adjusted to the focus of the blue rays ; but this is not so easy to find. After it has been found approximately, it is corrected by taking different photographs of a star, changing the position of the plate each time. The point is determined at which the best and sharpest picture is obtained, and, by continual repetitions of the attempts, the chemically active focus of a lens of 13 feet focal length can be accurately determined to within t^q of an inch. It is well known that all heavenly bodies have the same focus, on account of their great distance.
Xo photographic objective gives a correct picture with a large surface. Accordingly, to obtain the accuracy required by astronomy, the surface to be devoted to the image can only be very small, or about lj degrees in diameter. Any remaining distortions are controlled and corrected by photographing a very correct scale, and comparing the picture with the original. A field of 1J degrees, or three times the moon's diameter, embraces the well-known constellation of the Pleiades.
Rutherford's telescope was arranged as in fig. 116 ; it was moved by clockwork, so as to exactly follow the movement of the stars.
The views of large stars taken with it, after a short exposure, all appear as small round points which can