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Roberval was one of those mathematicians who, just before the invention of the infinitesimal calculus, occupied their attention with problems which are only soluble, or can be most easily solved, by some method involving limits or infinitesimals, and in the solution of which accordingly the calculus is always now employed.

"He brought into the administration," said Napoleon, "the spirit of the infinitesimals."

- The parabola affords a simple example of the use of infinitesimals.

The first volume treats of the analysis of finite quantities, and the second of the analysis of infinitesimals.

Each of the four co-ordinates, n', x', y are functions of %, rj, x, y; and if it be assumed that the field of view and the aperture be infinitely small, then, n, x, y are of the same order of infinitesimals; consequently by expanding ', ii', x', y in ascending powers of E, rt, x, y, series are obtained in which it is only necessary to consider the lowest powers.