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LIGHT-HORSE HARRY

1068

LIGHTHOUSE

4. Light travels with a finite speed. This was first proved by RSmer, the Danish astronomer,  at  Paris  during   1675-76. The fact that, when a landscape is illuminated at night by a flash of lightning, all parts are seen apparently at the same time, led the ancients to think that the speed of light was infinite. R5mer found the speed in a vacuum, i. e., in the space between us and the sun, to be 309 million meters per second. Professor Michelson's determination of this quantity, the most accurate determination which has  been made,  gives   299,853,000 meters per second. In 1850 Foucault, the French physicist, showed that light travels more slowly in all kinds of matter than it does in a vacuum. In water its speed is only three fourths as great as in ordinary glass and two thirds as great as in a vacuum.

5. In the first years of the i9th century Thomas Young of London showed that two rays of light might be added together in such a way as to produce darkness; in other words, that two rays might interfere. (See INTERFERENCE.)    Young's   experiment   is most easily repeated by holding immediately in front of the eye a visiting card (or, better, a piece of photographic plate) on which are cut two very fine slits about J millimeter apart. In 1( Coking at any small source of light, the beams coming through these two slits will be so diffracted as to overlap; and where  they  overlap,   they  will  sometimes interfere to produce brightness and sometimes    interfere    destructively,    producing darkness.    The result is that through the card one sees a series of alternate bright and dark bands.

6. Newton   (1643-1727)   showed   that   a ray of white  light  is composed  of many colors — what we call the colors of the rainbow, viz., red, orange, yellow, green, blue and violet. This he   accomplished by the use of two prisms.

7. In 1669 Bartolinus, a Danish philosopher, discovered that a ray of light which has   passed   through  a  crystal of  Iceland spar behaves very differently from ordinary light. First of all, it is split into two other rays,  which   emerge  from  the   crystal   in slightly^ different directions. If one of these transmitted rays be allowed to pass through a second crystal of Iceland spar, the effect produced  depends  very  much  upon  how the second crystal is held; it depends, in fact, upon the angular position of the second crystal   considered   with   reference   to   the incident   ray   as   an   axis. Rays  of  light having this property are said to be polarized.

NATURE OP LIGHT

Any idea of light which is at all satisfactory must explain at least the seven fundamental phenomena which have just been described. There are many other phenomena which are fundamental and which must also be explained by any sat-

isfactory view of the subject; space, however, prevents their introduction here. It is the great merit of Huygens, Young and Fresnel to have shown that, if we assume light to be a transverse wave-motion of the ether, all these seven phenomena can be easily explained. (See ETHER, HUYGENS, FRESNEL and YOUNG.) Since no other hypothesis has been offered that will explain these phenomena, we conclude that probably light is a wave-motion of the ether. For details of the explanation of these seven phenomena in terms of the wave-theory see Preston's Theory of Light. For applications of the principles of light to various optical instruments see LENS, SPECTROSCOPE, EYE, CAMERA, TELESCOPE, MICROSCOPE. For a beautiful elementary treatment see S. P. Thompson's Light, Visible and Invisible, and Lommel's Nature of Light (International Science Series.) For the physiological effects of light see Bidwell's Curiosities of Light and Sight.

HENRY CREW.

Light-Horse Harry. See LEE, HENRY.

Lighthouse, a building erected on some conspicuous part of the seacoast, from which a light is shown at night to guide mariners, and which serves as a landmark by day. A sea-light is thus defined by Alan Stevenson, the noted lighthouse architect: "A light so modified and directed as to*present to the mariner an appearance which shall at once enable him to judge of his position during the night in the same manner as would the sight of a landmark during the day." The history of lighthouse construction and illumination covers over 2,000 years; but the modern plan of construction dates back no further than the beginning of the 19th century. The first lighthouse tower of which we have record was that built by Ptolemy Philadelphus on a small island in the bay of Alexandria about 300 B. C. This- structure was deemed by the ancients one of the Seven Wonders of the World, and the name of the island, Pharos, was given to all lighthouses built by them. The Romans built lighthouses at Ostia, Ravenna, Puteoli and other ports; but none of the early lighthouse buildings is in existence. On the cliff at Boulogne are the remains of a lighthouse ascribed to Caligula (40 A. D.), and at Dover may be seen the remains of another Roman pharos. Cor-douan, at the mouth of the Garonne, has seen all the improvements, from the open grate in which wood and coal were burned to the dioptric light combined with a four-wick lamp. There were only 25 lighthouse stations in England at the beginning of the 19th century, but at present there are 1,000 coast and harbor lights. Some of the more notable lighthouses round the British Isles are the Eddystone, Skerry-vore, Bell Rock, Wolf and Bishop's Rock In the United States the first act of Congress