THE HELIOGRAPHIC DEPARTMENT
So far the development of the Observatory had been along the central line of assistance to navigation. But the Magnetic Department led on to one which had but a very secondary connection with it.
A greatly enhanced interest was given to the observations of earth magnetism, when it was found that the intensity and frequency of its disturbances were in close accord with changes that were in progress many millions of miles away. That the surface of the sun was occasionally diversified by the presence of dark spots, had been known almost from the first invention of the telescope; but it was not until the middle of the present century that any connection was established between these solar changes and the changes which took place in the magnetism of the earth. Then two observers, the one interesting himself entirely with the spots on the sun, the other as wholly devoted to the study of the movements of the magnetic needle, independently found that the particular phenomenon which each was watching was one which varied in a more or less regular cycle. And further, when the cycles were compared, they proved to be the same. Whatever the secret of the connection, it is now beyond dispute that as the spots on the sun become more and more numerous, so the daily swing of the magnetic needle becomes stronger; and, on the other hand, as the spots diminish, so the magnetic needle moves more and more feebly.
This discovery has given a greatly increased significance to the study of the earth's magnetism. The daily swing, the occasional 'storms,' are seen to be something more than matters of merely local interest; they have the closest connection with changes going on in the vast universe beyond; they have an astronomical importance.
And it was soon felt to be necessary to supplement the Magnetic Observatory at Greenwich by one devoted to the direct study of the solar surface; and here again that invaluable servant of modern science, photography, was ready to lend its help. Just as, by the means of photography, the magnets recorded their own movements, so even more directly the sun himself makes register of his changes by the same agency, and gives us at once his portrait and his autograph.
This new department was again due to Airy, and in 1873 the 'Kew' photo-heliograph, which had been designed by De la Rue for this work, was installed at Greenwich.
THE DALLMEYER PHOTO-HELIOGRAPH.
In order to photograph so bright a body as the sun, it is not in the least necessary to have a very large telescope. The one in common use at Greenwich from 1875 to 1897, is only four inches in aperture and even that is usually diminished by a cap to three inches, and its focal length is but five feet. This is not very much larger than what is commonly called a 'student's telescope,' but it is amply sufficient for its work.