A twenty-one inch reflector allows the rays issuing from it to diverge fifteen degrees. So that we have the light of the 360 degrees (the whole of the circle) gathered into fifteen (a twenty-fourth part of the circle). It does not quite follow that within that area the light will be twenty-four times as strong as if allowed to dissipate itself all round, because something must be allowed for absorption and waste; but we believe this allowance has been greatly overstated, and that where there are no mechanical difficulties in the way, the reflecting system is decidedly the best. Of course where it is necessary to light more than fifteen degrees of the circle, it will be necessary to use more reflectors, placing them side by side round a shaft, and if these are set into revolving motion, focus after focus of each reflector comes before the eye of the mariner, and the effect is all that can be desired. The dioptric or refracting system of lighting is the reverse of this. In the reflector the light is caught into a basin and thrown out again. In the refracting system, in its passage through the glass prisms, it is bent up or down and falls full upon the eye of the mariner, instead of wasting itself among the stars or down among the rocks at the lighthouse foot. For light, falling upon glass at a certain angle, does not go straight on, but gets deflected and transmitted in an altered line, as it does through water. And here comes the weakness of the dioptric system, in close vicinity to its strength. It is true that prisms and lenses send the light in the direction which is desired, but they charge a toll for the transmission; the glass is thick, and somewhat of the nature of a sponge. If we write on blotting-paper the marks appear on the other side, but some of the ink has soaked sideways, and there is very little doubt, that when light is transmitted through glass, a good deal of it is absorbed and retained.