Chronoscope

Chronoscope (time-observer), an invention of Sir Charles Wheatstone for determining the duration of luminous phenomena, e.g. a flash of light or the electric spark. Its main feature is a mirror which is in rapid motion. The speed of projectiles is sometimes measured by its means, and it has also been used to measure the time of passage of light through water and air respectively, in order to determine the proportionate degree of refraction.