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316 The Arabs constructed similar instruments, and already Mashallah, who lived about the year 775 (before the time of Al Mamun), wrote about astrolabes and armillæ, and these were used by Ibn Yunis, Abul Wefa, and others.

Alhazen also made use of armillæ for his investigations on refraction, and it has even been assumed that he is the inventor of the far simpler equatorial armillæ, which are generally ascribed to Tycho Brahe, who also considers himself as their inventor. But in any case, it is certain that equatorial armillæ were not known in Europe; that Walther, the principal observer before Tycho, only knew zodiacal armillæ; and that the principle of equatorial ones was first described in 1534 by Gemma Frisius, who, however, only designed an instrument of very