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_. [Illustration: 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. The _revolving light_ is produced by the revolution of a three or four sided frame, having large reflectors grouped on each side, with their axes parallel; and as the revolution exhibits once a minute, or once in two minutes, a light gradually increasing to the maximum, and then just as gradually decreasing to total darkness, its effect is remarkably impressive. The _revolving red and white_ is obtained by the revolution of a frame whose different sides present red and white lights, and exhibit the following succession:—two white lights after one red, or two red lights after one white.