Chronoscope

Chronoscope, an instrument contrived by Sir Charles Wheatstone to measure the duration of certain short-lived luminous phenomena, such as the velocity of light, or the electric spark, of which the eye itself can be no judge, owing to the persistence of impressions on the eye after the cause of sensation has ceased. The phenomenon is observed by reflection in a mirror in such rapid motion that the image of the luminous object would appear to describe a circular arc the length of which must be a measure of the duration of the light. The electric spark is found by this test to have no duration, because its image in the mirror is a mere point. The chronoscope has also been used for measuring the time of flight of projectiles. By means of it Foucault even determined the difference of the velocity of light when passing through air and water, and thus deduced the corresponding indices of refraction.