Complex sentence: Despite this limited path length, Foucault was able to measure the position of the displaced image of the slit with considerable accuracy, and his figure for the speed of light (298000 km/s) is within 0.6% of the modern value.
Split into simpler sentences: Despite this limited path length, Foucault was able to measure the position of the displaced image of the slit with considerable accuracy. Foucault's figure for the speed of light (298000 km/s) was within 0.6% of the modern value.