The material hitherto employed in the construction of lighthouse apparatus is crown glass, which, although it possesses a lower refractive power than flint glass and has, besides, a slightly greenish tinge, offers the great practical advantages of being more easily obtained of homogeneous quality; and, being less subject to deterioration from atmospheric influences, it is peculiarly suitable for use in the exposed situations generally occupied by Lighthouses. The refractive index of crown glass, as already noticed, is about 1·5.
Any one may easily satisfy himself by a careful protraction of the angles of incidence and refraction, in the manner above described, as to the truth of the following general propositions resulting from those laws:—
1. A ray of light passing through a plate of some diaphanous substance such as glass, with parallel surfaces, suffers no change of direction, but emerges in a line parallel to its original path, merely suffering a displacement, depending on the obliquity of the incident ray, and the refractive power and thickness of the plate. The effect of this displacement is merely to give the ray an apparent point of origin different from the true one. This will be easily understood by the diagram ([fig. 49]), in which a b is a normal to the plate, whose surfaces x x and x′ x′ are parallel, r r r r shews the path of the ray, r r the displacement, and r′ the apparent point of origin resulting from its altered direction.
Fig. 49.
2. When a ray passes through a triangular prism a b c, the inclination of the faces a c and c b causes the emergent ray r′ to be bent towards a b, the base of the prism, in a measure depending on the inclination of the sides of the prism and the obliquity of the incident ray to the first surface.
Fig. 50.
3. When parallel rays fall on a concave lens, they will, at their emergence, be divergent. The section of the diaphanous body a b c d may be regarded as composed of innumerable frusta of prisms, having their apices directed towards the centre line x r; and the rays which pass through the centre, being normal to the surface, will be unchanged in their direction, while all the others will (as shewn in the [figure]) suffer a change of direction, increasing with their distance from the centre, owing to the increasing inclination of the surfaces of the lens as they recede from its axis.