The photographs taken with this instrument are of two kinds. Those for the great chart have but a single exposure, but this lasts for forty minutes. Those for the great catalogue have three exposures on them, the three images of a star being some 20 seconds of arc apart. These exposures are of six minutes', three minutes', and twenty seconds' duration, and the last exposure is given as a test, since, if stars of the 9th magnitude are visible with an exposure of twenty seconds, stars of the 11th magnitude should be visible with three minutes' exposure. Thus it will be seen that in three minutes an impression is got of many scores of stars, whose places it would require many hours to determine at the transit instrument. But the positions of these stars on the plate still remain to be measured. For this purpose a net-work of lines, at right angles to each other, is printed on the photograph before its development, and, after it has been developed, washed and dried, the distances of the stars from their nearest cross-lines are measured in the measuring machine. [Illustration: THE DRIVING CLOCK OF THE ASTROGRAPHIC TELESCOPE. (_Reproduced from 'Engineering' by permission._)] The measuring machine is constructed to hold two plates, one half its breadth higher than the other. In fact, in each of the two series of photographs the whole sky is taken twice, but the two photographs of any region are not simply duplicates of each other. The centre of each plate is at a corner of four other plates, and in the micrometer the stars on the quarter common to two plates are measured simultaneously. In this way will be carried out a great census of the sky that will exceed Flamsteed's ten thousand fold. And just as Flamsteed's was but the first of many similar catalogues, so, no doubt, will this be followed by others--not superseded, for its value will increase with its age and the number of those that follow it, by comparison with which it will prove an inexhaustible mine of information concerning the motions of the stars and the structure of the universe. There is a great difference between the work of the observer with the 'Astrographic Telescope,' as this great twin photographic instrument is called, and the work of the transit observer. The latter sees the star gliding past him, and telegraphs the instant that the star threads itself on each of the ten vertical wires in succession. The astrographic observer, on the other hand, sees his star shining almost immovably in the centre of his field, threaded on the two cross wires placed there, for the driving-clock moves the telescope so as to almost exactly compensate for the rotation movement of the earth. The observer's duty in this case is to telegraph to his driving-clock, when it has in the least come short of or exceeded its duty, and so to bring back the 'guiding star' to its exact proper place on the cross wires.