Still the science of optics made little progress till the law determining the path of a ray of light was made known, and the laws of refraction discovered. Refraction means that a ray is deflected from its straight course by its passage from one transparent medium to another of different density. The old philosophers found out the theory of sound, and they applied themselves to light. Newton said light consisted of minute particles emanating from luminous bodies. Huyghens and Euler opposed Newton’s theory of the emission of light; and it was not till the celebrated Thomas Young, Professor at the Royal Institution, grappled with the question that the undulating or wave theory of light was found out. He based his investigations upon the theory of sound waves; and we know that heat, light, and sound are most wonderfully allied in their manner of motion by vibration. But he was ridiculed, and his work temporarily suppressed by Mr. Brougham. Light, then, is a vibratory motion (like sound and heat), a motion of the atoms of our ether. But how is the motion transmitted? Sound has its medium, air; and in a vacuum sounds will be very indistinctly heard, if heard at all. But what is the medium of communication of light? It is decided that light is transmitted through a medium called _ether_, a very elastic substance surrounding us. The vibrations, Professor Tyndall and other philosophers tell us, of the luminous atoms are communicated to this ether, or propagated through it in waves; these waves enter the pupil of the eye, and strike upon the retina. The motion is thus communicated by the optic nerve to the brain, and then arises the great primary faculty, Consciousness. We see light, the waves of which, or ether vibrations, are transversal; air waves or sound vibrations are longitudinal.