When the drum is circular instead of being polygonal, the lenses are cylindrical and not annular; the luminous rays are uniformly distributed in the horizontal plane, and act—in a meridian section—in the same manner as those of the annular lenses.
Yet there is something more than lenses in a dioptric apparatus, for this reason, that the lamp does something more than illuminate the frame. The rays streaming below it vainly poured their light at the foot of the tower, and those which rose above it were diffused in the upper region of the atmosphere, and consequently, for all purposes of marine illumination, would have been useless, had not Fresnel conceived the idea of collecting, concentrating, and despatching them in the same direction as the lenses threw the others. This he effected by means of the cylindrical rings of glass which, above and beneath the lenses, cover over the framework, as it were, or make use of it as a base, in expanding themselves as they approach the centre of the apparatus.
DIAGRAM, ILLUSTRATING THE PROGRESS OF A LUMINOUS RAY IN A CATADIOPTRIC RING.
The subjoined illustration represents the progress of a luminous ray in one of the rings of glass, technically termed catadioptric rings. Issuing from the focus F at the summit of the angle formed by the lines G and I, it is refracted at A in the direction A B, undergoes a complete reflection on the surface M N, takes the direction B C, and finally emerges from the ring in the horizontal line C H.
At this solution of the difficulty Fresnel did not arrive all at once, owing to the absolute want of workmen suitable for carrying out the novel industry which his genius had created. But by degrees these were trained and perfected; and the inventor had, moreover, the good fortune of discovering in an able and ingenious optician, M. Soleil, an efficient assistant in the construction on a large scale of the novel instrument he required. Afterwards the erection of lighthouses becoming an important branch of industry, he completed several edifices, which prospered all the more that strangers immediately gave up any attempt at rivalry, and left to him the work of supplying every maritime nation with lenticular apparatus.
Having said thus much of the central lens and its concentric rings of glass, a few words become necessary in reference to the lamp which feeds them, as it were, with light. Fresnel’s lamp may be shortly described as containing four concentric burners, which are defended from the excessive heat produced by their own combined flames by a superabundant supply of oil. This oil is pumped up from a cistern below by means of a clockwork movement, and overflows the wicks incessantly. To supply fresh currents of air to each wick with a rapidity sufficient to support the combustion, a very tall chimney-tube is found requisite. And yet the wicks do not carbonize with the extreme speed that might be supposed. It is even found, we are told, that after they have suffered a good deal, the flame does not perceptibly decrease, because the intense heat evolved from its mass encourages the rising of the oil in the cotton. Mr. Stevenson informs us that he has seen the large lamp in the Tour de Corduan burn for seven hours, and yet the wicks were neither snuffed nor raised. In the Scotch lighthouses a full flame is often maintained, with Colza oil, for no less a period than seventeen hours, and yet the lamp is untouched.