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ASTRONOMICAL PHOTOGRAPHY 305
the star catalogues, for the plates can be exposed under favourable circumstances and their examination carried out at leisure.
Photographs of the Moon. — But another heavenly body invites us specially to study it by the help of photography ; that is, our nearest neighbour, the moon. The unassisted eye recognizes its uneven surface (" mountains in the moon ") and the varying shades of its ground (moon spots). Its surface appearing as a rigid, almost vitreous, waterless, airless waste, contains a thousand problems.
Warren de la Rue tried to take photographic pictures of this singular body, which is so near to our earth and yet so different ; he succeeded in obtaining, with the help of a telescope, a small view of the moon, which he enlarged to 24 inches diameter with an enlarging apparatus.
The moon gives out less light than the sun. It is therefore taken to the best advantage in the principal focus of the telescope. In the most favourable case, three-quarters of a second sufficed for exposure, but it was rare to obtain sharp negatives, owing to the disturbance of the atmosphere, and to obtain a sharp image of the moon was a test of patience. After Warren de la Rue, Rutherford was celebrated for his moon-pictures ; his improved telescope, set up expressly for photographic purposes, gave a still sharper image of the moon than that of De la Rue.
Some years ago Schmidt, at Athens, maintained that an extinct volcano observed by Madler is no longer to be found, and he thereby proved the possibility of changes on the apparently rigid surface of the moon. If a photograph of the moon's surface could have been taken when Madler observed the volcano, we should now be certain about this point, which is still doubtful.
Spectrograj)hy. — But the sun and its eclipses, the u