Note: Do not rely on this information. It is very old.
Undulatory TheoryUndulatory Theory. Newton accounted for the effect of light by supposing that any source of light threw off tiny material particles in every direction, and that these, hitting against the retina, produced the sensation of light, i.e. caused us to see the luminous body. This hypothesis was known as the Corpuscular Theory, and held ground for some time; but it was shown that a result of this theory would be that light would travel faster in very refractive substances than in less refractive ones; it would, for example, have a greater velocity in water and air. Now experiment proves this to be untrue; light travels faster in air and in water and other highly refractive substances. So that theory broke down, and the Undulatory Theory has been accepted in its stead, it having been found to lead to conclusions consistent with actual experiment. According to this theory, a certain highly elastic medium is supposed to exist everywhere in space. The densest solid is permeated with it, as well as the rarest gas or the farthest realms of airless space. this medium is known as the luminiferous ether, agitation throws it into a state of vibration, and the vibration may be propagated in waves of different lengths, traveling with different velocities. Any luminous body is continually sending out these ethereal waves in every direction; those that enter the eye disturb the ether there, and produce an effect on the retina which causes us to see. If on its way away a wave meets with a plate of glass, the ether in the glass itself takes up the vibrations and hands them on to the other side; this is the case with all transparent substances. Opaque substance is are unable to pass the ethereal waves on, and hence the light is stopped. The eye, however, is not affected by all the vibrations capable of being transmitted by the ether; only those of certain wave-length may cause any sensation to be carried by the optic nerve. The waves which we call light-waves are extremely short, varying from .00003933 to 00007604 centimeters, according to their colour, those of blue being shorter than those of red light. Shorter waves still rush through the ether and produce definite effects, though the eye cannot detect them. Phosphorescent substances can grasp them, and photography eagerly picks them out. Longer waves we feel as heat, and the waves of electrical disturbance may be many miles in length. Maxwell supposed that all these ethereal waves were the effect of electromagnetic disturbance, and his theory, though incomplete at the time of his death, is being gradually elaborated, and is found to explain in the most satisfactory way many of the observed phenomena in optical science.
Click for more about Undulatory