Although Galileo eloquently championed mathematical methods in science, the main target of his wit and sarcasm in The Assayer was Grassi, a fellow astronomer, whose mathematical methods proved that comets move above the Moon. Galileo countered that comets are an optical illusion, produced by reflected light, so it becomes a meaningless question to ask where they are located. This copy, with expanded errata, is the usual state of the first edition.