in 1786, was to substitute reflectors in the room of the coal-light then in use at the Isle of May in the Frith of Forth, which, along with the light on the Cumbrae Isle in the Frith of Clyde, had, till that period, been the only beacons on the Scotch coast. The first reflectors employed in Scotland were formed of _facets_ of mirror glass, placed in hollow paraboloïdal moulds of plaster, according to the designs of the late Mr THOMAS SMITH, the Engineer of the Board, who (as appears from the article _Reflector_, in the Supplement to the third edition of the Encyclopædia Britannica) was not aware of what had been done in France, and had himself conceived the idea of this combination. The same system was also adopted in Ireland; and in time, variously modified, it became general wherever lighthouses are known. ~Reflection.~ To enable us to enter on the subject of the proper forms of reflectors, we must glance very briefly at the _laws of reflection_. Those laws are two in number. _1st_, The ray which falls on a reflecting surface, called the _incident_ ray, and the ray which leaves the reflector, called the _reflected_ ray, are always in one _plane_, which plane is perpendicular to the _reflecting surface_. _2d_, The angle which the _reflected_ ray makes with the reflector is always equal to the angle which the _incident_ ray makes with it, or, in other words, the angle of _incidence_ is equal to the angle of _reflection_.[41] [41] This will be more readily understood by referring to the accompanying figure (No. 22), in which CDEF is the reflecting surface; GHOKI the plane of reflection perpendicular to that surface; BO a line perpendicular or _normal_ to the surface CDEF; and AO the incident ray. Then if in the plane GHOKI, the angle BOI be made equal to AOB, OA′ is the reflected ray; BOG is then the angle of incidence; and BOI the angle of reflection. GOH and IOK, which are the complements of those angles, are, indeed, more strictly speaking, the angles of incidence and reflection; but in cases where the reflecting surface is curved, it is more convenient to refer the angles to the normal BO. [Illustration: Fig. 22.]