Complex sentence: During his imprisonment, in the years 1812--1814, he wrote his most notable work, "Traité des propriétés projectives des figures", which outlined the foundations of projective geometry, as well as some new results.
Split into simpler sentences: During his imprisonment, in the years 1812--1814, he wrote his most notable work; "Traité des propriétés projectives des figures". This work outlined the foundations of projective geometry, as well as some new results.