The book entitled "Assayer" published in 1623 was a controversial text about the theory of comets between Galileo and his opponent Horatio Grassi, the Jesuit mathematician. Galileo was an author of the original but false optical theory of comets while Grassi proposed a supra-lunar origin of comets supporting Tycho's opinion on comets and their motion. Unfortunately, Grassi's theory was based on a misunderstanding of how the telescope actually works., Vladimír Štefl., and Obsahuje bibliografii