It cannot be doubted that Blaeu owed to his abode on the island of Hveen the real foundation of his scientific knowledge, both in the field of geography and of astronomy, as well as his knowledge of the construction and the skilful use of mathematical instruments. We have reason for believing that a number of the instruments which served the great astronomer in his investigations, were the work of Blaeu, and it is an interesting fact, as we know, that Brahe’s observations, here made, formed the basis for Kepler’s calculations, leading him to the discovery of the laws which immortalized his name. It was perhaps late in the year 1596, or early in 1597, that Blaeu returned to Amsterdam where he soon established himself as a maker of mathematical instruments, of maps and of globes, and as an engraver and printer. Before the close of the seventeenth century, the Netherlands had moved into the first rank of European states in which the art of engraving and of printing flourished. Nothing perhaps contributed more to this end, especially in the north, than the freedom which there prevailed untrammeled by the restraints of the Index. As for the particular art of map engraving and map printing, the work of Mercator, of Ortelius, of Waghenaer, had centered the attention of those interested in this field upon the Low Countries. As early as 1541, not to mention his work of the preceding decade, Mercator issued his terrestrial globe gores, which mark a decided advance in the art of map engraving, and this was followed by his large map of England, of Europe, and of the World, the latter setting forth the new method of projection since known as Mercator’s projection. Ortelius’s great work of 1570, with its numerous succeeding issues, Waghenaer’s Mirror of Navigation of 1585, and Mercator’s book of maps of 1585, for which collection as a title the name Atlas was first used, were superior productions, models for those who followed closely within the same field. Blaeu acknowledged these men to be his masters, though, as was also frequently the case with other map makers and map engravers of his day, he occasionally forgot to assign proper credit to those from whom he borrowed in the cartographical work that issued from his press.