When one of the lenses came opposite the observer, the eye received a bright flash preceded and followed by a short eclipse. Before and after the eclipses the fixed light was visible. This arrangement is called a "fixed light varied by flashes." Nearly the, same appearance is given by using the upper and lower prismatic rings of the fixed light and the annular lenses of the revolving light. When the latter revolves, the eye perceives a flash from the annular lens, and in the interval between the flashes perceives the light from the fixed part of the apparatus. It will be seen that the variations which this system admits for the same order far surpass in number those of reflector lights. The latter can only be either fixed or revolving, and it has been found impossible in practice to diversify the intervals between the flashes to any extent. The radii of the spherical surfaces forming the lenses, and the radii and other dimensions of the prismatic rings, are calculated by known formulas from the index of refraction, the position of the exterior surfaces of the lens with reference to the source of light, and the distances of the various concentric rings and prismatic surfaces from the horizontal plane passed through the principal focus.

The surfaces of the lenses are limited in breadth by the condition that the solid of revolution shall not be thick enough to absorb a material portion of the light. This condition makes their breadth about 1.5 in. Spherical aberration is nearly eliminated by a proper use of the formulas in calculating the radii of the surfaces. The index of refraction of the glass used is 1.51. In the large lenses the rings are ground in segments of circles, are fastened into brass armatures, and are put together at the lighthouse. The small lenses are placed in their armatures, and are put together, ready for erection, at the workshops. The material used is the flint glass of St. Go-bain, which, although not as colorless as crown glass, was selected because it could be obtained more free from bubbles and striae than crown glass. It is cast in pieces, exceeding the intended size of the finished parts by about one eighth. There are six orders of lenses, arranged according to size. The three first and largest are used in seacoast lights, and the three last in harbor and river lights, and generally in those of lesser importance.

Fresnel's invention has also been adapted to small lanterns used for steamers' signal lights, pier-head and ferry lights, etc, and many of this kind are now manufactured in the United States of pressed glass. The first order fixed lens apparatus is about 6 ft. in diameter and 9 ft. high. The central zone consists of the central plano-convex belt and 16 steps (echelons), arranged in equal numbers above and below it. The lower set of prisms is 6 in number, and the upper set 13. This last set is arranged in the form of a cone. In the revolving first order lens, having an interval of 10 seconds, there are 17 upper and 8 lower prisms. The sixth and smallest order of lens is 11.8 in. in diameter and 17.5 in. high. The central zone is composed of the plano-convex belt and four steps, two on each side of it. There are three prisms below and five above the central zone. As the lamps in use when Fresnel made his invention were entirely incompetent to supply enough light from one burner for the use of the higher orders of lens apparatus, he in conjunction with Arago made a thorough investigation of the subject of lamps. The result was that he adopted for the first order lens a burner about 3 1/2 in. in diameter, giving a flame about 4 1/3 in. high, and containing four concentric wicks.

The'intensity of the light of this lamp is about equal to that of 25 ordinary Carcel burners which have a diameter of about three fourths of an inch. The lamp is placed in the centre of the apparatus. As the heat evolved by such a lamp is very great, there might be danger of melting the burners, and of burning up the wicks. To avoid these difficulties, Fresnel adopted the Carcel lamp, which, by a system of clockwork, pumps up to the burner four times as much oil as is consumed. By this means the burners are always kept comparatively cool, and the wicks sometimes burn a whole night without requiring snuffing. For the second order lens apparatus a lamp with three concentric wicks was adopted; for the third and fourth orders, lamps with two concentric wicks are used; and for the fifth and sixth orders, ordinary Argand burners are used. Very slight changes in any of the details of the lamps have been made since they were first settled by Fresnel. The annual con-sumption of oil by the lenses of the different orders is as follows: first order, 084 gallons; second, 461; third, 221; fourth, 150 to 190, according as one wick or two are used; fifth, 70; sixth, 60. In the first order octagonal revolving lens the quantity of light sent to the horizon by one of the octagonal faces and its reflectors is between 3,000 and 4,000 times the light of a single Carcel burner, being eight times as much as that sent to the horizon by the best reflectors that are made.

To get the useful effect of the whole lens, the above numbers must be multiplied by 8, that being the number of annular lenses doing the work of that number of burners at the same time. The useful effect of the lens light is to that of the reflector light as 4 to 1; that is, one gallon of oil burned in a lens light throws as much light to the horizon as four gallons burned in a reflector light. The brilliancy of a first order lens light as compared with the best reflector lights is as 83 to 10, or as 5 to 1.-The first lens apparatus manufactured under the direction of Fresnel was erected in 1823 in the Cordouan lighthouse at the mouth of the Gironde, on the coast of the bay of Biscay. The auxiliary lenses and reflectors for utilizing the rays above and below the central belt are now replaced by the prismatic rings. In 1825 the lens system was adopted for the coasts of France, and as early as 1838, 12 lighthouses on the coast were illuminated by the Fresnel system. In 1845 there were 151 lens lights on the French coast, and probably there is not a single reflector light in France at present. It was next adopted by the Dutch, and in 1834 the erection of a first order lens in Inchkeith (Scotland) lighthouse was authorized.

The Trinity house corporation next adopted the Fresnel system in 1837, and it has since been used by all European maritime nations and their colonies, and by the United States. (See Lighthouse.)