A ray of light, when transmitted _obliquely_ from one transparent body to another of different density, undergoes, at the point where it strikes the common surface of the two planes, a sudden change of direction. This change of direction is called _refraction_. For instance, plunge one half of a straight ruler into a basin of water. The ruler no longer appears straight, but _bent back_ or _broken_ (_re_, and _fractum_) at the point where it enters the water. We have already stated that the great object to be gained in lighthouse illumination is this very _refraction_; that is, the rays of the lamp must be refracted, or bent back, so as to strike and illuminate the sea. Fresnel saw that this object might be secured by the employment of lenses to intercept, as it were, and refract the rays proceeding from the lamp. What kind of lens possessed the greatest power of refraction? He preferred the plano-convex lenses, which, instead of having two curve surfaces, have one surface a curve, and the other a plane. And the lens thus adopted he built up in separate pieces, for the still greater economization and intensity of light; and, says Mr. Stevenson, he has subdivided with so much judgment the whole surface of the lens into a centre lens and concentric annular bands, and has so carefully determined the elements of curvature for each, that it seems unlikely any improvement will soon be effected in their construction. [Illustration: ANNULAR LENS OF FIRST ORDER.] The central disc of the lens, marked B in the accompanying diagram—as employed in lights of the first order—is about 11 inches in diameter, and the focus distance equals 920 millimètres, or 36.22 inches. The annular rings surrounding it gradually decrease in breadth as they recede from the centre, from 2¾ to 1¼ inches. The lens, we should add, is made of crown glass. A lens of this magnitude costs about £60. Its weight is about 109 lbs., and its surface consists of about 1300 square inches; but though composed of so many parts, it is held together simply by two narrow strips of polished glass, united by a thin film of cement. * * * * * The following illustration, representing a segment of the profile of a dioptric apparatus, will give the reader a sufficient idea of the manner in which the rays proceeding from the focus of a lamp are refracted on issuing from the lens; it also shows the central disc, and the rings placed above and below it. When the drum is circular instead of being polygonal, the lenses are cylindrical and not annular; the luminous rays are uniformly distributed in the horizontal plane, and act—in a meridian section—in the same manner as those of the annular lenses.