Although, in drawing his own character, Kepler describes himself as "troublesome and choleric in politics and domestic matters," yet the general events of his life indicate a more peaceful disposition than might have been expected from the peculiarities of his mind and the ardour of his temperament. On one occasion, indeed, he wrote a violent and reproachful letter to Tycho, who had given him no just ground of offence; but the state of Kepler's health at that moment, and the necessitous circumstances in which he had been placed, present some palliation of his conduct. But, independent of this apology, his subsequent conduct was so truly noble as to reconcile even Tycho to his penitent friend. Kepler quickly saw the error which he committed; he lamented it with genuine contrition, and was anxious to remove any unfavourable impression which he might have given of his friend, by the most public confession of his error, and by the warmest acknowledgments of the kindness of Tycho. In his relations with the scientific men of his own times, Kepler conducted himself with that candour and love of truth which should always distinguish the philosopher. He was never actuated by any mean jealousy of his rivals. He never scrupled to acknowledge their high merits; and when the discoveries made by the telescope established beyond a doubt the errors of some of Kepler's views, he willingly avowed his mistake, and never joined in the opposition which was made by many of his friends to the discoveries of Galileo. A striking example of this was exhibited in reference to his supposed discovery of Mercury on the sun's disc. In the year 1607,[50] Kepler observed upon the face of the sun a dark spot, which he mistook for Mercury; but the day proving cloudy, he had not the means of determining by subsequent observations whether or not this opinion was well founded. As spots on the sun were at that time unknown, Kepler did not hesitate to publish the fact in 1607, in his _Mercurius in Sole visus_; but when Galileo, a few years afterwards, discovered a great number of similar spots with the telescope, Kepler retracted his opinions, and acknowledged that Galileo's discovery afforded an explanation, also, of many similar observations in old writers, which he had found it difficult to reconcile with the actual motions of Mercury. [50] It is said that Kepler saw this dark spot _while looking at the sun in a camera obscura_. As a camera obscura is actually a telescope, magnifying objects in proportion to the focal length of the lens employed, he may be said to have first seen these spots with the aid of an optical instrument.