able questions that meet us on the threshold of every science. Some philosophers entertain the opinion that light consists of tiny particles of matter thrown off from a luminous body with prodigious velocity in all directions. Others suppose it to be an undulation or vibration produced in a medium called ether, which is believed to pervade all space. The former view of the subject is termed the theory of emission; the latter, the undulatory theory. We cannot say which of these hypotheses, or guesses, approaches nearest to the truth, but the undulatory theory has by far the greater number of supporters.
Whether a ray of light be a stream of inconceivably minute particles of matter, or a succession of waves in an ethereal medium, we are quite certain that it travels at the rate of nearly two hundred thousand miles in a single second. But such is the disproportion between the distances of the celestial bodies, that light must be about eight and a quarter minutes in reaching us from the sun; about five hours in coming from the planet Neptune; years from the nearest fixed star; and probably centuries from the nebulæ! When we look up at the heavens, we do not see the stars as they are now, but as they were many years ago, for the light which now renders them visible must have left them long before we were born!
Rays of light are emitted, under ordinary circumstances, in direct lines; they will not pass
