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136 continued to be sent backwards and forwards between Cassel and Uraniborg, in which were discussed the methods of observing, the instruments in use, and, after the publication of Tycho's system of the world, also the question whether this system or that of Copernicus was the true one. We shall in the sequel have many opportunities of quoting these letters, or rather astronomical essays, of which Tycho recognised the interest to the scientific world by sending copies of some of them to several other correspondents, and finally by publishing them all in a volume printed at Uraniborg, which forms an excellent supplement to his other writings, and completes the picture of his scientific activity.

All the details about Tycho's observatory which the Landgrave had learned from Tycho's letters to himself and Rothmann had naturally made him anxious to see it for himself, and an opportunity of doing so seemed to offer itself in 1588, as there was to be a meeting of North German princes at Hamburg, which the Landgrave was going to attend. King Frederick had already given orders to have ships ready to carry the Landgrave over to Seeland, when the king's death prevented the meeting at Hamburg, and with it a second meeting of Tycho and the Landgrave.

But though Wilhelm IV. never came to Hveen, Tycho had from time to time the pleasure of welcoming other distinguished guests at Uraniborg. Among these we shall here mention Johan Seccerwitz, professor in Greifswalde, who is known as a Latin poet. He came to Denmark in 1580 with the Duke of Pomerania to attend the christening