The constant denoting the velocity of light in vacuum is encountered in every branch of physics, and this uni- versality brings out the unity of the physical world. Although it is now over three hundred years since the constant was first defined, it yielded to scientific assaults only slowly, revealing as it did unexpectedly new phenomena. Its universality and the surprises that it threw up make any attempt to relate how the velocity of light came to be measured a minor history of physics. This book therefore oxplains the requisite science against the back- ground of the historical personalities involved. Intended for teachers and school pupils.