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LIGHTHOUSE ILLUMINATION.
disc of light thrown by the apparatus upstairs, that there is nothing like the amount of aberration that there would have been if it had been one great bull's-eye. Here is one of Fresnel's lamps of the fourth order so constructed (fig. 57):
Fig. 57.
observe the fine effect obtained by these different lenses as you see them revolve before you, and understand that all this upper part is made to form part of the lens, each prism throwing its rays to increase the effect, and, although you may think it is imperfect because if you happen
