Cosmo II., not a little alarmed by these gloomy prognostications of his ambassador, and really in care for the revered philosopher, at length issued the order for his long-desired return. Picchena then wrote the following drastic letter to Galileo, on 23rd May:— “You have had enough of monkish persecutions, and know now what the flavour of them is. His Highness fears that your longer tarriance at Rome might involve you in difficulties, and would therefore be glad if, as you have so far come honourably out of the affair, you would not tease the sleeping dog any more, and would return here as soon as possible. For there are rumours flying about which we do not like, and the monks are all powerful. I, your servant, would not fail to warn you, and to inform you, as in duty bound, of the wishes of our ruler, wherewith I kiss your hand.”[158] Galileo complied without delay with Cosmo’s wishes, and set out on his homeward journey on the 4th of the following month. CHAPTER VIII. _THE CONTROVERSY ON COMETS._ Studious Seclusion.—Waiting for the Correction of the Work of Copernicus.—Treatise on Tides.—Sends it to Archduke Leopold of Austria.—The Letter which accompanied it.—The three Comets of 1618.—Galileo’s Opinion of Comets.—Grassi’s Lecture on them.—Guiducci’s Treatise on them inspired by Galileo.—Grassi’s “Astronomical and Philosophical Scales.”—Galileo’s Reply.—Paul V.—His Death.—Death of Cosmo II.—Gregory XV.—“Il Saggiatore” finished.—Riccardi’s “Opinion” on it.—Death of Gregory XV.—Urban VIII.