Page:Optics.djvu/182

 length of the waves must be equal to the distance that the light passes over whilst the vibration takes place; since therefore the waves are found to be shorter in the more strongly refracting substances, this velocity of transmission must be less in them according to the same law; that is to say, it must be inversely as the ratio of refraction.

By considering the alternations of light and darkness as produced by the superposition of luminous waves of the same or of different nature, we give to the phænomenon a physical character, and it is thus that Dr. Young first announced the principle of interferences; but we may detach it, as he has done, from all extraneous considerations, and present it as an experimental law; it may then be expressed as follows:
 * When two equal portions of light, in exactly similar circumstances, have been separated, and coincide again nearly in the same direction, they either are added together, or destroy one another, according as the difference of the times, occupied in their separate passages, is an even or odd multiple of a certain half interval which is different for the different kinds of light, but constant for each kind.
 * In the application of this law to different media, the velocities of light must be supposed to be inversely proportional to the ratios of refraction for those media, so that the rays move more slowly in the more strongly refracting medium.
 * In reflexions at the surface of a rarer medium, on some metals, and in some other cases, half an interval is lost.
 * Lastly, it may be added that the length of this interval, for a given kind of light, is exactly four times that of the fits attributed by Newton to the same light.

To give an instance of these laws, suppose that when two simple homogeneous rays interfere and form fringes in the experiment with the two mirrors, you interpose across the path of one of these a very thin plate of glass that that ray alone is to pass through. According to the second condition, its motion through the glass must be slower than through the air in proportion as the refracting power is greater. Thus, when after leaving the glass,