Then descending once more to the floor from which we started, we see that among the columns there is a large clockwork arrangement, the purpose of which is to drive round the turntable and all that it carries--in the language of the lighthouse engineer the "optical apparatus" or, more briefly, "the apparatus." And as this turns the radiating beams of light sweep round the horizon and in succession strike into the eyes of any mariner who may be within range. Each time a beam strikes him he sees a flash. If the apparatus revolve once a minute he will see four flashes every minute, one from each panel. Let us consider, then, the advantages of this wonderful mechanism, with its cunning arrangement of prisms. It is these latter, of course, which are the important thing. The rest, the mechanical portion, is simply for the purpose of holding them and turning them at the proper speed. In the first place, the contrivance gives us flashes instead of a steady light; it gives the lighthouse its "character." Then again it enhances the brightness of the light. Instead of shining all round, the light is concentrated in four special directions, and the light which would be wasted upwards or downwards is saved and brought into use. But suppose that the lighthouse we are considering be near the shore, so that there is no need for it to throw any light in one--the landward--direction. Then we should see inside the revolving framework with its prisms a fixed frame with reflecting prisms which would catch any rays going from the lamp in the direction of the land and simply hurl them, as it were, back into the flame. Thus the intensity of the flame becomes increased by those rays thrown back which would else have been wasted. Or suppose that the character of the light is such that the flashes have to be at irregular intervals. Then the framework, instead of being symmetrically four-sided, would be of an irregular shape. And that brings us to a beautiful feature of the mechanism of the apparatus. We have been discussing a four-panel arrangement. Suppose that we were to reduce it to three. Then, since all the light would be concentrated into three beams instead of four, each beam would be more intense. We should thereby have increased the range of our apparatus without any increase in the cost of oil--for nothing, as it were. But to get the same number of flashes per minute we should have to drive it round so much the faster. But increased speed means increased burden on the keepers who have to wind up the heavy weights which operate the clockwork. So there is a limit to the speed which can be attained.