Later this year, the Planetary Society is planning to launch LightSail-1, a sail that will travel farther and farther into space propelled by the sun's steady stream of photon particles. Other such flights are projected for the next few years. This renewed interest in light and its momentum call for a fresh look into the speed of light c which generates this energy. This speed has been, for over a century, a cardinal pillar supporting the present edifice of theoretical physics, but that light had any speed at all was discovered only in the seventeenth century, and confirmed in the eighteenth, by means of measurements over vast astronomical distances. These methods seem to have proven that the speed of light varied with the speed of the observer. Nonetheless, another experiment late in the nineteenth century, was interpreted as showing that the speed of light was independent of its observer or its source-it was a universal constant. This thesis is a meticulous examination in historical context of the evidential data and theories which paved the road to the idea that the speed of light was a universal constant, a seemingly universal belief. The need for the endeavor arose secondarily to the main task of exploring fundamental physical and geometrical phenomena of light detailed in the previous treatises on Optokinetics.

Later this year, the Planetary Society is planning to launch LightSail-1, a sail that will travel farther and farther into space propelled by the sun's steady stream of photon particles. Other such flights are projected for the next few years. This renewed interest in light and its momentum call for a fresh look into the speed of light c which generates this energy. This speed has been, for over a century, a cardinal pillar supporting the present edifice of theoretical physics, but that light had any speed at all was discovered only in the seventeenth century, and confirmed in the eighteenth, by means of measurements over vast astronomical distances. These methods seem to have proven that the speed of light varied with the speed of the observer. Nonetheless, another experiment late in the nineteenth century, was interpreted as showing that the speed of light was independent of its observer or its source-it was a universal constant. This thesis is a meticulous examination in historical context of the evidential data and theories which paved the road to the idea that the speed of light was a universal constant, a seemingly universal belief. The need for the endeavor arose secondarily to the main task of exploring fundamental physical and geometrical phenomena of light detailed in the previous treatises on Optokinetics.