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.
So far, the work of the Astrographic Department has been, as mentioned above, a development on an extraordinary scale, but a development still, of the original programme of the Observatory. But the munificent gift of Sir Henry Thompson has put it within the power of the Astronomer Royal to push this work of sidereal photography a stage further. Sir Henry Thompson gave to the Observatory, not merely the photographic refractor of 9 inches' aperture, now used for solar photography, and known as the 'Thompson photo-heliograph,' but also one of 26 inches' aperture and 221/2 feet focal length. This instrument was specially designed of exactly double the dimensions of the standard astrographic telescope used for the International Photographic Survey, the idea being that, in the case of a field of special interest and importance, a photograph could be obtained with the larger instrument on exactly double the scale given by the smaller. It has rather, however, found its usefulness in a slightly different field. The observation of the satellites of Jupiter was suggested by Galileo as a means of determining the longitude at sea. As already pointed out, the suggestion did not prove to be a practical one for that purpose, but observations of the satellites have been made none the less with a view simply to improving our knowledge of their movements, and of the mass of Jupiter. The utilitarian motive for the work having fallen through, it has been carried on as a matter of pure science.
And the work has not stopped with the satellites of Jupiter; eight satellites were in due time discovered to Saturn, four to Uranus, and two to Mars; and though these could give not the remotest assistance to navigation, they too have been made the subjects of observation for precisely the same reason as those of Jupiter have been.
THE THOMPSON TELESCOPE IN THE NEW DOME.
In just the same way, when the discovery of Neptune was followed by that of a solitary companion to it, this also had to be followed. The difficulties in the way of observing the fainter of all these satellites were considerable, and the work has been mostly confined to two or three observatories possessing very large telescopes. As the largest telescope at Greenwich was only 7 inches in aperture up to 1859, and only 123/4 inches up to 1893, it is only very recently that it has been able to take any very substantial part in satellite measures. But since the Thompson photographic telescope was set up, it has been found that a photograph of Neptune and its satellite can be taken in considerably less time than a complete set of direct measures can be made, whilst the photograph, which can be measured at leisure during the day, gives distinctly the more accurate results.
So, too, the places of the minor planets can be got more accurately and quickly by means of photographs with this great telescope than by direct observation, and photographs of the most interesting of them all, the little planet Eros, have been very successfully obtained. So that, though doing nothing directly to improve the art of navigation, or to find the longitude at sea, the great photographic refractor takes its share in the work of 'Rectifying the Tables of the Planets.'