488 FRESNEL tinction was formed by revolving a system of cylindrical vertical lenses around a fixed ap- paratus and outside of it. These lenses col- lected the rays incident upon them, and emit- ted them parallel to each other and to the ho- rizon. When one of the lenses came opposite the observer, the eye received a bright flash preceded and followed by a short eclipse. Be- fore and after the eclipses the fixed light was visible. This arrangement is called a "fixed light varied by flashes." Nearly the same ap- pearance is given by using the upper and lower prismatic rings of the fixed light and the annu- lar 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 re- flector lights. The latter can only be either fixed or revolving, and it has been found im- possible 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 pris- matic 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 sur- faces 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 1-61. 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 to- gether, 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 in- tended size of the finished parts by about one eighth. There are six orders of lenses, ar- ranged 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 inven- tion has also been adapted to small lanterns used for steamers' signal lights, pier-head and ferry lights, &c., 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-con- vex belt and 16 steps (echelons), arranged in equal numbers above and below it. The low- j er set of prisms is 6 in number, and the up' i per 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 IT upper and 8 lower prisms. The sixth and smallest order of lens is 11 '8 in. in diam- eter and IT'S 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 in- vention were entirely incompetent to supply enough light from one burner for the use of the higher orders of lens apparatus, he in con- junction with Arago made a thorough investi- gation of the subject of lamps. The result was that he adopted for the first order lens a burner about 3 in. in diameter, giving a flame about 4J- 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, Fres- nel adopted the Carcel lamp, which, by a sys- tem of clockwork, pumps up to the burner four times as much oil as is consumed. By this means the burners are always kept compar- atively 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 con- centric wicks are used ; and for the fifth and sixth orders, ordinary Argand burners are used. Very slight changes in any of the de- tails 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, 684 gallons; second, 461 ; third, 221 ; fourth, 156 to 190, according as one wick or two are used ; fifth, TO ; 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 num- bers 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 re- flector light. The brilliancy of a first order lens light as compared with the best reflector lights is as 83 to 16,- 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
