Complex sentence: The random character of stock market prices was first modelled by Jules Regnault, a French broker, in 1863 and then by Louis Bachelier, a French mathematician, in his 1900 PhD thesis, "The Theory of Speculation".
Split into simpler sentences: The random character of stock market prices was first modelled in 1863 by Jules Regnault, a French broker. Later, it was modeled in 1900 by a French mathematician, Louis Bachelier, in his 1900 PhD thesis, "The Theory of Speculation".