Scheiner, in his "Appelles post Tabulam," describes four different ways of viewing the spots; one of which is by the _interposition of blue or green glasses_. His first method was to observe the sun near the horizon; the second was to view him through a transparent cloud; the third was to look at him through his telescope with a blue or a green glass of a proper thickness, and plane on both sides, or to use a thin blue glass when the sun was covered with a thin vapour or cloud; and the fourth method was to begin and observe the sun at his margin, till the eye gradually reached the middle of his disc. On the publication of Scheiner's letters, Velser transmitted a copy of them to his friend Galileo, with the request that he would favour him with his opinion of the new phenomena. After some delay, Galileo addressed three letters to Velser, in which he combated the opinions of Scheiner on the cause of the spots. The first of these letters was dated the 4th of May 1612;[23] but though the controversy was carried on in the language of mutual respect and esteem, it put an end to the friendship which had existed between the two astronomers. In these letters Galileo showed that the spots often dispersed like vapours or clouds; that they sometimes had a duration of only one or two days, and at other times of thirty or forty days; that they contracted in their breadth when they approached the sun's limb, without any diminution of their length; that they describe circles parallel to each other; that the monthly rotation of the sun again brings the same spots into view; and that they are seldom seen at a greater distance than 30 deg. from the sun's equator. Galileo likewise discovered on the sun's disc _faculae_, or _luculi_, as they were called, which differ in no respect from the common ones but in their being brighter than the rest of the sun's surface.[24] [23] The original of this letter is in the British Museum. [24] See Istoria e Dimonstrazioni, intorno alle macchie solare. _Roma_, 1616. See Opere di Galileo, vol, v., p. 131-293.