_Kepler's Pecuniary Embarrassments--His Inquiries respecting the Law of Refraction--His Supplement to Vitellio--His Researches on Vision--His Treatise on Dioptrics--His Commentaries on Mars--He discovers that the orbit of Mars is an Ellipse, with the Sun in one focus--And extends this discovery to all the other Planets--He establishes the two first laws of Physical Astronomy--His Family Distresses--Death of his Wife--He is appointed Professor of Mathematics at Linz--His Method of Choosing a Second Wife--Her Character, as given by Himself--Origin of his Treatise on Gauging--He goes to Ratisbon to give his Opinion to the Diet on the change of Style--He refuses the Mathematical Chair at Bologna._ Although Kepler now filled one of the most honourable situations to which a philosopher could aspire, and possessed a large salary fitted to supply his most reasonable wants, yet, as the imperial treasury was drained by the demands of an expensive war, his salary was always in arrear. Owing to this cause he was constantly involved in pecuniary difficulties, and, as he himself described his situation, he was perpetually begging his bread from the Emperor at Prague. His increasing family rendered the want of money still more distressing, and he was driven to the painful alternative of drawing his income from casting nativities. From the same cause he was obliged to abandon his plan of publishing the Rudolphine Tables, and to devote himself to works of a less expensive kind, and which were more likely to yield some pecuniary advantages.