Page:Astronomy for Everybody.djvu/239

reader that the companion of Saturn made its revolution in fifteen days. A copy of this was sent to Wallis, the celebrated English mathematician. In his reply the latter thanked Huyghens for his attention, and said he also had something to say, and gave a collection of letters longer than that of Huyghens. When the latter interpreted his anagram to Wallis, he was surprised to receive in reply a solution of the Wallis anagram announcing the very same discovery, but, of course, in different language and at greater length. It turned out that Wallis, who was expert in ciphers, wanted to demonstrate the futility of the system, and had managed to arrange his own letters so as to express the discovery, after he knew what it was. Huyghens did not appreciate the joke.

The Paris Observatory was founded in 1666 as one of the great scientific institutions of France which adorned the reign of Louis XIV. Here Cassini discovered the division in the ring, showing that the latter was really composed of two, one outside the other, but in the same plane. The outer of these rings has somewhat the appearance of being again divided, by a line called the Encke division, after the astronomer who first noticed it, but the exact nature of this division is still in doubt. It certainly is not sharp and well defined like the Cassini division, but only a slight shade.

To understand the varying appearance of the rings we give a figure showing how they and the planet would look if we could see them perpendicularly (which we