Page:Various Forces of Matter.djvu/176

164, and if it does not diverge at all, it makes the light so small, that perhaps only one in a hundred can see it at the same time. The South Foreland lighthouse is, I think, 300 or 400 feet above the level of the sea, and therefore it is necessary to have a certain divergence of the beam of light in order that it may shine along the sea to the horizon. I have drawn here two wedges, one has an angle of 15°, and shows you the manner in which the light opens out from this reflector seen at the distance of half a mile or more, the other wedge has an angle of 6°, which is the beautiful angle of Fresnel. When the angle is less than 6°, the mariner is not quite sure that he will see the light—he may be beneath or above it; and in practice it is found that we cannot have a larger angle than 15°, or a less one than 6°. In order, therefore, to get more light, we must have more combustion, more cotton, more oil; but already there are in that lamp four wicks put in concentric rings, one within the other, and we cannot increase them much more, owing to the divergence which would be caused by an increase in the size of the light—the more the divergence, the more the light is diffused and lost. We are,