The reflectors, as used in the best lighthouses, are made, says Mr. Stevenson,[19] of sheet copper, plated in the proportion of six ounces of silver to sixteen ounces of copper. They are moulded to a paraboloidal form by a delicate and laborious process of beating with mallets and hammers of various forms and materials, and are frequently tested during the operation by the application of a mould carefully formed. After being brought to the curve, they are stiffened round the edge by means of a strong bizzle, and a strap of brass which is attached to it for the purpose of preventing any accidental alteration of the figure of the reflector. Polishing powders are then applied, and the instrument receives its latest finish.
To prove the form of the reflector, two gauges of brass are employed. One is for the back, and used by the workmen during the process of hammering; the other—while the mirror undergoes its final touches—is applied to the concave face. The mirror is then tested by trying a burner in the focus, and measuring the intensity of the light at various points of the reflected conical beam.
The flame generally used in reflectors is derived from an Argand lamp, with wicks an inch in diameter. The burners are sometimes tipped with silver to prevent the wick from being wasted by the great heat which is evolved. They are also fitted, in many of the Scottish lighthouses, with a sliding apparatus of accurate shape, by which they can be removed from the interior of the mirror at cleaning time, returned exactly to the same place, and locked by means of a key.
Catoptric lights, we may add, are divided into nine separate classes, differing in some respect from those recognized by the French authorities. The nine classes are called fixed, revolving white, revolving red and white, revolving red with two whites, revolving white with two reds, flashing, intermittent, double fixed lights, and double revolving white lights.
AN ARGAND FOUNTAIN LAMP.
The following account of the distinctive character of each class of light is condensed from a valuable treatise by Mr. Alan Stevenson:—
The fixed exhibits a regular and steady appearance, and is not subject to any change; and the reflectors employed are smaller than those required for revolving lights. This is necessary, in order that they may be ranged round the circular frame, with their axes so inclined as to admit of their illuminating every point of the horizon.