They did not look through or into the telescopes.

The lens was turned upon the object, and this was thrown, enormously magnified, upon screens of what looked something like ground glass some fifty feet square.

It was thus that they saw, not only the whole visible surface of Jupiter as he revolved above them and they about him, but also their native Earth, sometimes a pale silver disc or crescent close to the edge of the Sun, visible only in the morning and the evening of Jupiter, and at other times like a little black spot crossing the glowing surface.
But there was another development of the science of the Crystal Cities which interested them far more than this--for after all they could not only see the Worlds of Space for themselves, but circumnavigate them if they chose.
During their stay they were shown on these same screens the pictorial history of the world whose guests they were.

These pictures, which they recognised as an immeasurable development of what is called the cinematograph process on Earth, extended through the whole gamut of the satellite's life.

They formed, in fact, the means by which the children of Ganymede were taught the history of their world.
It was, of course, inevitable that the _Astronef_ should prove an object of intense interest to their hosts.