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80 to assist him, and the work at his observatory had for a long time made little or no progress, when Tycho Brahe arrived at Cassel in the beginning of April 1575. The Landgrave was well pleased to receive the young astronomer as his guest, and they conversed by day about their favourite science, and observed the heavens by night together, the Landgrave with his own quadrants and unwieldy torqueta, Tycho with some portable instruments, among which was probably his sextant. Among other observations they determined the position of Spica Virginis. Naturally they discussed the nature and position of the new star, and the Landgrave told Tycho how he had once been so intent on determining the greatest altitude of the star, that he had not even desisted when he was told that part of the house was on fire, but had calmly finished the observation before leaving the observatory. Tycho was also interested to learn that the Landgrave had remarked how the motion of the sun became retarded when it approached the horizon at sunset, which might be seen by watching a sun-dial. Tycho recollected having read the same in the observations of Bernhard Walther (before whom, however, Alhazen had recognised in this phenomenon an effect of refraction), and he determined to follow up the matter by-and-by, so as to be able to correct observations made at low altitudes for refraction.

More than a week had elapsed in thus exchanging ideas and opinions, when a little daughter of the Landgrave died, and Tycho, who did not wish to intrude his company on the