A curious application of this principle is shown in Fig. 200. A lens is arranged with its axis in the meridian, so that its principal focus shall fall upon the vent of a small cannon

When the sun

crosses the meridian, the rays are concentrated upon the vent, and if the gun has been loaded and primed beforehand, it will be discharged at midday.

Light-houses.

304. LIGHT-HOUSES are towers, erected along the coast, upon the tops of which are lanterns. These lanterns are lighted at night as guides to mariners.

One of the most famous light-houses of antiquity was that on the little Island of Pharos, near Alexandria, in Egypt. From the location of this light-house the French derive the name pharo, which they apply to all light-houses. In former times light-houses were illuminated by fires built with wood, coal, or some bituminous substances.

Explain Fig. 200. (304.) What is a light-house? Give an account of the ancient light-houses.

These methods of illumination were afterwards replaced by oil lamps placed in the foci of concave reflectors, which served to concentrate the rays, and thus to heighten their illuminating effect. But the reflectors, being made of metal, were soon tarnished, and the light afforded became feeble.

In 1822, FRESNEL, already distinguished by his discoveries in optics, and by his researches on the wave theory of light, invented a new system of illumination, which is now being adopted in all civilized countries.

Abandoning the reflectors, which became tarnished by the influence of sea fogs, he substituted for them plano-convex lenses, in the principal foci of which he placed powerful lamps with four concentric wicks, each of which, for the quantity of oil consumed, and the amount of light given out, was found to be equivalent to seventeen carcel-lamps. The difficulty of constructing large plano-convex lenses, together with their great absorption of light, led finally to the adoption of a particular system of lenses, known as échelon lenses.

These lenses will be understood by examining Figs. 201 and 202;

Explain the principle of reflectors. What modification did FRESNEL introduce? Explain the échelon lens.

Fig. 201 shows a front view, and Fig. 202 a section or profile of an échelon lens.

A lens of this kind consists of a plano-convex lens, A, about a foot in diameter, around which are disposed several annular lenses, which are also plano-convex, and whose curvature is so calculated that each one shall have the same principal focus as the central lens, A.

A lamp, L, being placed at the principal focus of this refracting system, as shown in Fig. 202, the light emanating from it is refracted into an immense beam, RC, of parallel rays.

Besides this refracting system, several ranges of reflectors, mn, are so disposed as to reflect such light as would otherwise be lost, to increase the beam of light formed by refraction.

By this double combination, an immense beam of light is afforded, which renders the light visible for fifteen or twenty leagues; but this beam is only visible in a single direction. To remedy this defect, FRESNEL united eight systems similar to that just described, which combination presents the appearance of a pyramid of glass, nine or ten feet in height.

Fig. 203 represents a section of the lantern of a light-house of the

first order, which was actually constructed by M. SAUTTER, and exhibited at the great "Universal Exposition" of France, in 1855. In order to illuminate all points of the horizon, the system is made to revolve on a vertical axis by clock-work. The clock-work is shown at M in the figure, and the weight at P. To prevent friction the system turns upon six wheels, or rollers, shown in the figure to the left of M.

In consequence of this rotation an observer at any point will see eight flashes of light during one revolution, which are followed by as many intervals of darkness, called eclipses. By suitably regulating the number of revolutions in any given time, different light-houses may be distinguished from each other.

Fig. 204 shows a complete light-house, and the manner in which it throws out a beam of light. The distance at which these lights may be seen depends upon the quality of the illuminating apparatus, and upon their altitude above the sea. They are usually built upon - bluffs, or else the tower is sufficiently elevated to place the lantern from 150 to 200 feet above the level of the sea. The United States government is engaged, through its present efficient Light-house Board, in constructing a great number of light-houses on our coast. of the most approved description.

IV. DECOMPOSITION OF LIGHT.COLORS OF BODIES.

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Solar Spectrum.

305. When a ray of solar light passes through a prism, it is not only deviated, but it is decomposed into several rays, which are scattered, or spread apart.

The property which a refractive medium possesses of decomposing and scattering solar light, is called its dispersive power, and the phenomenon is called dispersion.

The phenomenon of dispersion is shown in Fig. 205. A beam of light entering a hole in the shutter of a darkened

How are all points of the horizon illuminated? What are flashes? Explain their production. Explain Fig. 204. (305.) How does a prism act to scatter rays? What is the dispersive power? Dispersion? Illustrate.