Sir Isaac Newton objected to the theory of undulations because experiments seemed to show that light could not travel through bent tubes, which it ought to do if propagated by undulations like sound; and it was reserved for the late Dr. Young to prove that light could and would turn a corner, in his highly philosophical experiments illustrating the inflection or bending in of the rays of light. Dr. Young placed before a hole in a shutter a piece of thick paper perforated with a fine needle, and receiving through it the diverging beams on a paper screen, found that when a slip of cardboard one-thirtieth of an inch in breadth was held in such a beam of light, that the shadow of the card was not merely a dark band, but divided into light and dark parallel bands, and instead of the centre of the shadow being the darkest part, it was actually white. Dr. Young ascertained that if he intercepted the light passing _on one side_ of the slip of card with any opaque body, and allowed the light to pass freely on the other side of the slip of cardboard, that all the bands and the white band in the centre disappeared, and hence he concluded that the bands or fringes within the shadow were produced _by the interference [Page 330] of the rays bent into the shadow by one side of the card, with the rays bent into the shadow by the other side_. (Fig. 315). [Illustration: Fig. 315.] In order to show how two waves may interfere so as to exalt or destroy each other, two sets of waves may be propagated on the surface of a still tank or bath of water, from the two points A A (Fig. 315), the black lines or circles representing the tops of the waves. It will be seen that along the lines B B the waves interfere just half way between each other, so that in all these directions there will be a smooth surface, provided each set of waves is produced by precisely the same degree of disturbing force, so as to be perfectly equal and alike in every respect, and the first wave of one set exactly half a wave in advance of the first wave of the other, while at the curve in the direction of all the line C C, the waves coincide, and produce elevations or undulations of double extent; in the intermediate spaces, intermediate effects will, of course, be produced. Professor Wheatstone has invented some very simple and beautiful acoustic apparatus for the purpose of proving that the same laws of interference exist also in sound, which, as already stated, consists in the vibrations or undulation of the particles of air. [Page 331] The nature and effects of interference are also admirably illustrated by the following models of Mr. Charles Woodward, President of the Islington Scientific Institution, and to whom we have already alluded.