Page:On the various forces of nature and their relations to each other.djvu/184

180 to avoid by building up compound lenses in the manner here shewn (fig. 58). Look at this beautiful apparatus—is it not a most charming piece of workmanship? Buffon first, and Fresnel afterwards, built up these kind of lenses, ring within ring, each at its proper adjustment, to compensate for the effects of spherical aberration. The ring round that centre lens is ground so as to obviate what would otherwise give rise to spherical aberration; and the next ring being corrected in the same manner, you will perceive, if you look at the 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): 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 to sit below or above the horizontal line, you perceive but little if any of the light, yet you must bear in mind