ered one of the principal ideas of his philosophy. It was in this
period that his first independent discovery in mathematics was
made, namely, the so-called Euler theorem on polyhedrons
e + f = k + 2.

Weary of army camps, Descartes left the military service
shortly after the Battle of the White Mountain ( Nov. 8, 1620)
and the occupation of Prague. He enlarged his knowledge of the
world and of men through extended travel over Hungary, Ger-
many and Italy ( 1621-25). Returning to Paris once again, he
became an intimate friend of Mersenne ( 1588-1648) whose
scientific friends were taking a stand against Aristotelian natu-
ral philosophy with visible success. Descartes found lively in-
terest in this circle with respect to his novel points of view. He
stayed for a short time at the camp near the Huguenot base, La
Rochelle, where he became acquainted with the military engi-
neer, Desargues. Then, in 1628, in a sudden decision, he returned
to the Netherlands. He worked there in almost completely with-
drawn solitude, on the formal construction of his system, keep-
ing in touch only with Mersenne, out of the whole scientific
community of Paris. In 1631, he went to England for a brief
period; in 1634, to Denmark. He engaged in a correspondence
on philosophy (from 1643) with Princess Elizabeth (a daughter
of the Winter King) and it was for her sake that be went to
Paris in 1644, 1646 and 1648. In 1644, he made the acquaint-
ance of Chanut, the French ambassador in Stockholm. In 1647,
the latter was the intermediary in the correspondence between
Descartes and Queen Christina of Sweden. In 1649, Descartes
accepted an invitation to visit the Queen, who wished to become
familiar with his philosophy. Christina had under contempla-
tion the establishment of an academy of sciences but Descartes
died before the plans for the academy were carried out. His liter-
ary remains were brought to France by Chanut. Though the pa-

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