Kater's pendulum minutes per day slower than at Paris, or equivalently the length of a pendulum with a swing of one second there was 1¼ Paris "lines", or 2.6 mm, shorter than at Paris. It was realized by the scientists of the day, and proven by Isaac Newton in 1687, that this was due to the fact that the Earth was not a perfect sphere but slightly oblate; it was thicker at the equator because of the Earth's rotation. Since the surface was farther from the Earth's center at Cayenne than at Paris, gravity was weaker there. Since that time pendulums began
Glan–Foucault prism Glan–Foucault prism A Glan–Foucault prism (also called a Glan–air prism) is a type of prism which is used as a polarizer. It is similar in construction to a Glan–Thompson prism, except that two right-angled calcite prisms are spaced with an air gap instead of being cemented together. Total internal reflection of "p"-polarized light at the air gap means that only "s"-polarized light is transmitted straight through the prism. Compared to the Glan–Thompson prism, the Glan–Foucault has a narrower acceptance angle over which it works, but because it uses an air gap rather than cement, much higher irradiances can be used