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ASTRONOMICAL PHOTOGRAPHY 287
The accompanying figure shows a telescope of this kind adapted to photographic purposes. The objective, 0, remains in its place ; the eye-piece, which would be fixed at the other end of the tube, is taken away, and an apparatus, V (Fig. 116), is substituted for it, which is identical with the hinder part of a photographic camera. It contains a ground-glass slide, S, which, after the image has been focussed, can be exchanged for a sensitive plate. The focussing is effected by a rackwork motion at T.
But now a difficulty occurs through the motion of the stars, which necessitates the telescope following this movement. For this purpose, the stand of the telescope is furnished with a driving-clock, which causes it to revolve in the direction of the course of the stars, so that the telescope is what is called equatorially mounted. Fig. 117 shows an arrangement of this kind.
The oblique support of the telescope resting on the foot is parallel to the axis of the earth. The polar axis of the telescope is turned completely round in this support once in twenty-four hours.
The telescope, d d, is not fixed immediately on this axis, but on an axis c, at right angles to it ; it can be turned round the latter (the declination axis) in all directions perpendicular to the axis, c i. The movement of the two axes allows any star to be brought into the field of the telescope.
Historical Survey. — When Arago, in August 1839, announced to the French Academy of Sciences the invention of Daguerre, he also proposed that this method of obtaining pictures should be made use of for photographing the moon, and also for getting more complete representations of the solar spectrum than could then be made in any other way.
However, only one daguerreotype appears to have been taken of the sun in France for a period of about twenty years. This was taken by Fizeau and Foucault in 1845.