A new world was opened for him when he made the acquaintance of A. Bernstein's comprehensive popular books on scientific subjects. This work is looked on nowadays as being somewhat antiquated and, in the eyes of many a professional scientist, has sunk to the level of a pseudo-scientific "shocker"; even when Einstein as a boy made explorations in it, there were signs of rust and decay in the work, for it originated in the fifties of the previous century and, in point of subject-matter, had long been transcended. Yet it could be read then--and even now--as a story containing thousands of interspersed physical, astronomical, and chemical wonders, and for the boy Einstein it came to be a true book of Nature, which presented to his mind, greedy for knowledge, as much as it did to his imagination. Other vistas were opened up to him by Büchner's _Kraft und Stoff_ (_Force and Substance_), a book the cheapness of which he could not yet discern, but which called up wonder in him without rousing his criticism. In addition, his attention was chiefly occupied by a handbook of elementary planimetry, containing an abundance of geometrical exercises, which he fearlessly attacked and within a very short time solved almost in their entirety. His delight grew when he ventured into the difficulties of analytical geometry and infinitesimal calculus quite apart from the curriculum of his school-work. Lübsen's textbook had fallen into his hands, and these directions sufficed for his audacious spirit. Whereas many of his school companions were still standing undecidedly before the pools of theorems of congruence and repeating decimals, he was already disporting himself freely in the ocean of infinitesimals. His work did not remain concealed, and gained appreciation. His mathematical teacher declared that the fifteen-year-old boy was ripe for university study.