Huyghens, on the contrary, supposes light to be caused by the waves or vibrations of an infinitely elastic medium--ETHER--diffused through all space, which waves are propagated in every direction from the luminous body. In the first theory, a luminous particle is supposed actually to come from the sun to the earth; in the other, the sun only occasions a disturbance of the _ether_, which extends with great rapidity, in the same manner as a wave spreads itself over the surface of a lake. Nearly all the facts known in the time of Newton, and those discovered by him, were explained most satisfactorily by his hypothesis; but it was found they could be interpreted equally as the effects of undulation, with the exception of the production of colour by prismatic refraction. Although the labours of many gifted minds have been given, with the utmost devotion, to the support of the vibratory theory, this simple fact has never yet received any satisfactory explanation; and there are numerous discoveries connected with the molecular and chemical disturbances produced by the sun’s rays, which do not appear to be explained by the hypothesis of emission or of undulation. In both theories a wave motion is admitted, and every fact renders it probable that this mode of progression applies not only to light, but to the so-called _imponderable forces_ in general. Admitting, therefore, the undulatory movement of luminous rays, we shall not stop to consider those points of the discussion which have been so ably dealt with by Young, Laplace, Fresnel, Biot, Fraunhofer, Herschel, Brewster, and others, but proceed at once to consider the sources of light, and its more remarkable phenomena.[86] The sun is the greatest permanently luminous body we are acquainted with, and that orb is continually pouring off light from its surface in all directions at the rate, through the resisting medium of space and of our own atmosphere, of 192,000 miles in a second of time. It has been calculated, however, that light would move through a vacuum with the speed of 192,500 miles in the same period. We, therefore, learn that a ray of light requires eight minutes and thirteen seconds to come from the sun to us. In travelling from the distant planet Uranus, nearly three hours are exhausted; and from the nearest of the fixed stars each ray of light requires more than six years to traverse the intervening space between it and the earth. Allow the mind to advance to the regions of nebulæ, and it will be found that hundreds of years must glide away during the passage of their radiations. Consequently, if one of those masses of matter, or even one of the remote fixed stars, was “blotted out of heaven” to-day, several generations of the finite inhabitants of this world would fade out of time before the obliteration could be known to man. Here the immensity of space assists us in our conception, limited though it be, of the for-ever of eternity.[87]