We are in the topmost room of the lighthouse, the lantern, as it is called. In the centre there stands the murette or pedestal. In this several columns support a circular platform on the top of which there moves what we might call a turntable, which in turn bears a frame of gun-metal into which are fitted a maze of glass bars triangular in section and curved to form concentric circles. The whole structure, possibly, is of great size. From the floor to the platform is as high as an ordinary man. Indeed around the turntable there is a gallery which forms a roof over our heads, so that it is only after mounting some iron steps on to this gallery that we are able to examine the glass part. As we ascend we notice that the walls of the chamber as far up as the gallery are formed of iron plates, while above that there is a metal framework filled in with glass panes, and above all a dome-shaped roof. Having reached the platform we proceed to examine the glass, and we find that the metal framework forms a cage with four sides, each approximately flat, but really slightly spherical. Each of these sides is called a "panel." In the centre of each is a lens. Peeping through the interstices between the prisms, we perceive that the lamp is inside this structure, exactly in the centre, so that its light shines directly through the central lens or bull's eye. Around this bull's-eye are many circles of glass bar, forming refracting prisms. Around this again are more bars in the form of segments, which together form circles, some being refracting prisms and others reflecting prisms. All the light rays from the lamp which fall on any one prism are deflected, so that they proceed approximately in the same direction. Those prisms in the upper part lay hold of the rays which would otherwise go up into the sky. Those at the bottom collect those which would fall near the foot of the tower. So scarcely any are lost. But for the fact that the lamp itself is comparatively large and not a theoretical point, as already explained, the beam from this panel would be perfectly straight, parallel, and of uniform density everywhere. As it is, it widens slightly as it proceeds, but, practically speaking, we might call it a solid beam of light. Each of the panels sends forth such a beam, so that they strike out in four directions from the central lamp much as four spokes from the hub of a wheel.