Plate XIII. shews an elevation of a revolving dioptric apparatus of the first order; F is the focal point, in which the flame is placed; L, L great annular lenses, forming by their union an octagonal prism, with the lamp in its axis, and projecting, in horizontal beams, the light which they receive from the focus; L′L′, the upper lenses, forming by their union a frustum of an octagonal pyramid of 50° of inclination, and having their foci coinciding in the point F. They parallelise the rays of light which pass over the lenses. M, M are plane mirrors, placed above the pyramidal lenses L′L′, and so inclined as to project the beams reflected from them in planes parallel to the horizon; Z, Z are the lower zones, first used at Skerryvore, in the room of the curved mirrors which were used at Corduan. The lower part of Plate XIII. shews the moveable framework which carries the lenses and mirrors, and the rollers on which it circulates, with the clockwork which gives motion to the whole. Plate XIV. is the plan of the apparatus shewn in Plate XIII. Plate XV. shews a section of a fixed dioptric light of the first order. F is the focal point in which the flame is placed; R, R cylindric refractors, forming by their union a prism of thirty-two sides, or a true cylinder, with the lamp in its axis, and producing a zone of light of equal intensity in every point of the horizon; M, M, curved mirrors, ranged in tiers above and below the cylindric refractors, and having their foci coinciding in the point F; the effect of the mirrors increases the power of the light, by collecting and transmitting the rays which would otherwise pass above and below them, without increasing the effect of the light. Plate XVI. gives the elements of the curved mirrors MM of Plate XV. After the details given of the nature of the catadioptric zones, all that is needful is briefly to refer to Plate XVII., in which ABC and A′B′C′ shew the upper and lower zones which supply the place of the mirrors shewn at M, M, in Plate XV.; while DEF shews the cylindric belt as lately improved, with the diagonal joints M, N, C; and X, X, represent the diagonal supports for the cupola ABC. This plate, in connection with the enlarged view of the same apparatus at Plate XVIII., affords a complete explanation of the arrangement of all the parts. ~Arrangement of the Dioptric Apparatus in the Lightroom.~ For the purpose of arranging the various parts of the dioptric apparatus in their proper positions, _three_ gauges are employed. The first (fig. 88) is for ascertaining that the lenses _l_, _l_, meet at the proper horizontal angle, so that their axes shall meet with the proper inclination in F the focus. This is done by means of two arms, whose projecting points _r_, _r_, _r_, _r_ touch the backs of the lenses, while the graduated arc _c_ indicates the inclination of _l_, _l_, to _l_, _l_, or the complement of that inclination at F. [Illustration: Fig. 88.] [Illustration: Fig. 89.]