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Beacons.

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This property which condensing lenses have is utilised for producing fire in what are called burning glasses. They may be a source of danger, by becoming a source of fire, when a lens is exposed to the solar rays. The same accident may be produced by spherical glass vessels; for they refract the light and heat like double convex lenses.

The concentration of the heat rays of the sun has received an application in certain solar dials, when the hour of midday is marked by the discharge of a small cannon (fig. 255). Above the cannon is a condensing lens, the focus of which exactly corresponds to the touch-hole of the cannon the moment the sun passes the meridian of this place. Hence, the cannon being charged and primed beforehand, the lens ignites the powder just at midday, and the explosion announces the time at a distance.

Yet the time thus given is what is called in astronomy solar time, or true time, in which the length of day varies. Now our watches and clocks being regulated for mean time, that is to say, for an unchangeable day, only agree with the sun four times a year; December 24, April 15, June 15, and September 1. On February 11 a clock giving mean time is 14′ 37′′ faster than the sun, and on November 3 it is 16′ 17′′ slow. The equation of time represents the amount which on all the days of the year must be added to or taken from the time of a clock to obtain the mean time. Hence it is incorrect to use the ordinary expression, that a good watch or a good clock goes like the sun.

332. Beacons. Lighthouses.-These are fires lighted at night on high towers along the shores of the sea, in order to guide mariners in darkness and enable them to keep clear of danger.

Beacon fires were originally wood or coal fires; but these were dull and unsteady. They were afterwards replaced by oil lamps placed in the principal focus of concave reflectors, which sent the reflected light to a great distance, for its rays were parallel.

In 1822 Fresnel made a great improvement in the illumination of lighthouses as they are now called. Abandoning the use of metallic reflectors, which soon tarnished under the influence of the sea-fogs, Fresnel substituted large plano-convex lenses, in the focus of which he placed a powerful lamp with four concentric wicks, and equal in illuminating power and quantity of oil consumed to seventeen Carcel lamps. But the difficulty of constructing such lenses, which must necessarily be large, and at the same time not thick, so as not to absorb much light, led Fresnel to

adopt a special system of lenses, known as echelon or lighthouse lenses.

Seen in front in fig. 256, and in profile in fig. 257, they consist of a plano-convex lens, A, a foot in diameter, round which are arranged eight or ten glass rings, which are also plano-convex, and whose curvature is calculated, so that each has the same focus as the central lens, A. A lamp being placed in the focus of this refracting system, an immense horizontal pencil, RC, is formed, which sends the light to a great distance. Further, above and below these lenses, are placed several silvered glass mirrors, mn.

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Thus the rays, which would be lost towards the sky and the earth, are utilised and sent in a horizontal direction. By this double combination a vast horizontal pencil is obtained, which sends the light of the lamp to a distance of 20 or 30 miles; but it only sends it in one direction. To increase the number of points of the horizon at which the light can be seen, Fresnel, instead of a single system of lenses and mirrors represented in fig. 257, united eight such arrangements, so as to form an enormous glass pyramid with eight faces, as seen in fig. 258, which represents a lighthouse lens of the largest size, constructed by M. Soutter, and exhibited at the

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Lighthouse Lenses.

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Paris Universal Exhibition in 1855. The system of mirrors and lenses alone is 10 feet high.

A lighthouse lens of this kind sends a powerful beam of light towards eight points of the horizon, but all other points are desti

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tute of light, so that vessels sailing in these dark parts would have no help from the lighthouse. This difficulty was removed by Fresnel by means of a very simple mechanism, represented at the lower part of fig. 258. A clockwork motion, M, moved by a weight, P, imparts to the whole system of lenses, AB, a slow rotating motion on six rollers. During a complete revolution of the apparatus, the

whole horizon is successively illuminated, and the mariner lost in the night sees the light alternately appear and disappear after equal intervals of time. These alternations serve to distinguish lighthouses from an accidental fire or a star. By means too of the number of times the light disappears in a given time, and by the

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colour of the light, sailors are enabled to distinguish the lighthouses from one another, and hence to know their position.

Of late years the use of the electric light has been substituted for that of oil lamps; a description of the apparatus will be given in a subsequent chapter.

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Solar Spectrum.

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CHAPTER V.

DECOMPOSITION OF LIGHT BY PRISMS.

333. Solar spectrum.-In speaking of prisms and lenses, we have only considered the change in direction which these transparent media impart to luminous rays, and the images which result therefrom; but the phenomenon of refraction is by no means so simple as we have hitherto assumed when white light, or that which reaches us from the sun, passes from one medium into another, it is decomposed into several kinds of lights, a phenomenon to which the name dispersion is given.

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Fig. 26c.

In order to show that white light is decomposed by refraction, a pencil of solar light, SA (fig. 260), is allowed to pass through a

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