Again, for ascertaining the verticality of the main lenses, or for setting the subsidiary lenses or mirrors shewn in Plates XIII. and XIV., at the required angle of inclination, recourse is had to a clinometer (fig. 89) touching the back of the lens LL by means of studs at A, A, while the spirit-level S indicates, on the graduated limb, the amount of deviation from the vertical position of the instrument, whether accidental or intentional. [Illustration: Fig. 90.] Lastly, to test the true position of the lamp itself, with reference to the lenses thus properly arranged, we apply a radius or trainer (fig. 90) which fits into the centre burner at F, while its point A touches the centre of the lens _l_, _l_; at B is a graduated slide, which admits of the trainer being lengthened or shortened to suit various focal distances; and the spirit-level at _c_ at once corrects any error in the length of the trainer arising from depression or elevation, and also serves to indicate the proper level for the burner which is noticed at page 290, in speaking of the lamp. The dotted line F _c′_B′A′ shews the position of the trainer in reference to an adjacent lens. The elegant apparatus invented by AUGUSTIN FRESNEL for Harbour Lights, on the same principle as that just described for Sea Lights, is shewn beneath (fig. 91). It consists of thirteen rings of glass of various diameters arranged one above another, in an oval form. The five middle rings have an interior diameter of 11·81 inches (30^{cm.}), and like those of the larger apparatus, refract equally over the horizontal plane of the focus the light which they receive from it. The other rings or prisms, five of which are upper and three lower, are ground and set in such a manner, that they project all the light derived from the focus in a direction parallel to the other rays by _total reflection_. [Illustration: Fig. 91.] The arrangements of this apparatus, which is distinguished by the addition of external refractors arranged vertically, will be more fully understood by a reference to fig. 91, which shews its section and plan. F is the focal point in which the flame is placed; _r_, _r_ cylindric refractors, forming by their union a cylinder with a lamp in its axis, and producing a zone of light of equal intensity all round the horizon; _x_, _x_ are catadioptric prismatic rings acting by _total reflection_, and giving out zones of light of equal intensity at every point of the horizon. The dotted lines shew the course traversed by the rays of light which proceed from the lamp, and are acted upon by the rings of glass. The letters _r′_ _r′_ shew the external prisms, having their axes at right angles to those of the principal bent prisms, composing the refractors at _r_, _r_, and revolving around them. This ingenious application of the property of crossed prism is already described at page 264.