The Eighth Ray, also known as the anti gravity ray, is one of the rays of all forms of light. It propels the light of the sun or other light sources away from the source and towards other objects. Each planet has it's own unique eight ray that "reflects," or propels the light. This ray is constantly streaming out from the planet constituting a force of repulsion of gravity which when confined is able to lift enormous weights from the surface of the ground.
Excistance of this ray is widely known on Barsoom, and is is mainly used in air ships as a medium of buoyancy. Large amounts of the ray are stored in so called buoyancy tanks, allowing a flier to rise up from the ground.
Knowledge of the eighth ray eventually spread to Earth and it revolutionized travel on the planet.