After what has been already said on the subject of divergence, it will at once be seen, that in revolving lights the reflectors are placed with their axes parallel to each other, so as to concentrate their power in one direction; whilst in fixed lights it is necessary, in order to approach as near as possible to an equal distribution of the light over the horizon, to place the reflectors, with their axes inclined to each other, at an angle somewhat less than that of the divergence of the reflected cone. For this purpose, a brass gauge (see fig. 41), composed of two long arms, AM, AM, somewhat in the form of a pair of common dividers, connected by a means of a graduated limb A, is employed. The arms having been first placed at the angle, which is supplemental to that of the inclination of the axes of the two adjacent mirrors at O, are made to span the faces of the reflectors, one of which is moved about till its edges are in close contact with the flat surface of one of the arms of the gauge. [Illustration: Fig. 42.] [Illustration: Fig. 43.] Figs. 42 and 43 shew an elevation and plan of a revolving apparatus on the catoptric principle. In these figures, _n n_ shews the reflector frame or chandelier; _o o_, the reflectors with their oil-fountains _p_ _p_. The whole is attached to the revolving axis or shaft _q_. The copper tubes _r_ _r_ convey the smoke from the lamps; _s_ _s_ are cross bars which support the shaft at _t t_; _u u_ is a copper pan for receiving any moisture which may accidentally enter at the central ventilator in the roof of the light-room; _l_ is a cast-iron bracket, supporting the cup in which the pivot of the shaft turns; _m_ _m_ are bevelled wheels, which convey motion from the machine to the shaft. The machinery does not require any particular notice, being that of common clock-work, moved by the descent of a weight. [Illustration: Fig. 44.] Fig. 44 shews a plan of one tier of reflectors arranged in the manner employed in a fixed catoptric light; _n_ _n_ shews the chandelier, _q_ the fixed shaft in the centre, which supports the whole, _o_ _o_ the reflectors, and _p_ _p_ the fountains of their lamps. In this figure (in order to prevent confusion) only one tier of reflectors is shewn; the other tiers are so arranged, that their axes divide into equal angles the arcs intercepted between the axes of the adjoining reflectors on the first tier, thereby producing the nearest approach to an equal distribution of the light, which is attainable by this arrangement.