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 CHAPTER XII

of years ago the author suggested the use of a telescope of twelve or fifteen inches aperture and several hundred feet focal length for photographing the Moon. It was not possible to carry out the plan at that time, but early in the year 1900, owing to the generosity of two anonymous givers, an opportunity occurred which permitted the experiment to be tried. The amount of the donation permitted the use of a twelveinch (30 cm.) objective, and it was decided that the photographic focus should be 135 feet 4 inches (4,125 cm.), thus representing the object on a scale of exactly 5″ to one millimetre. It was obvious that with so long a focus the tube must be fixed in one position and a reflection of the object viewed in a movable mirror. It was later found best to reduce the aperture of the lens to six inches, and all save three of the negatives used in the atlas were taken with this reduced aperture.

An expedition to the island of Jamaica in 1899, during which a five-inch telescope was used at five different stations upon the island, had shown that the atmospheric conditions were extremely favourable to astronomical work during the summer season, and it was hoped that they would be equally so during the winter, which is the time of the greatest freedom from clouds. Later investigations showed that while the seeing in winter is good, yet it is decidedly inferior to that in the summer season. The latitude of the island, which is about 18° N., permitted the mounting of the telescope with the axis of the lens parallel to that of the Earth, the tube being placed upon the side of a hill, with the mirror and lens at the lower end. The mirror was supported in a steel fork, which was also placed with its axis parallel to that of the Earth. The mirror could be clamped at different angles with regard to the fork. The fork was caused to revolve about its axis once in twenty-four hours in the opposite direction to that in which it was turned by the Earth. It therefore had no angular motion with regard to the heavenly bodies. A similar rotation was given to the photographic plate.

This form of mounting does not permit the portion of the sky in the immediate vicinity of the Pole to be examined, but as the instrument was devised for use on the members of the solar system, this objection was of little consequence. The mirror was 89