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THREE-DIMENSIONAL MOTION PICTURES I fill all spaces in the universe. The ether must exist in the inter stellar spaces beyond the earth's atmosphere, so as to permit the light energy emitted by the sun and the stars to reach the earth, as well as it must fill the infinitesimally small interstices between the molecules of all substances from the lightest gas to the heaviest metals. Each point of a light source is supposed to become the center of a disturbance creating a spherical system of waves, which thrown into the ether travel at a velocity first calculated by Roemer to be approximately 186,000 miles per second. Each point of this spherical .system is supposed to become itself the center of a disturbance and thus create its own sys tem of secondary waves, thus forming an unlimited complex of spherical systems of waves within a sphere, whose center is the luminous point. The outmost boundary of the main sphere is called the front wave. Huygens presented his theory at the French Academy of Sciences in 1678 and through it gave a physical and mathemati cal explanation of nearly all known phenomena pertaining to light. The theory received the appellation of undulatory theory due to the undulatory progression of the waves. Newton challenged the truth of the undulatory theory in favor of the emission theory because Huygens could not sat isfactorily explain the rectilinear propagation of light. In later times, Fresnel, (1802) succeeded in proving the ap proximate rectilinear propagation of light by the wave theory and in expressing such a thorough explanation of the phenom enon of diffraction that it proved the ultimate argument in favor of this theory. The emission theory was then entirely discarded. Fresnel admitted the existence of the luniniferous ether stat ing that its density is constant within the same substance or medium but varies from medium to medium and, by consider ing the light waves as elastic waves, such as those visible in a taut cord set to vibrate, he could establish the law that the velocity of light in any medium is inversely proportional to the square root of the density of the ether inside that medium.