OF
A REFLECTOR
For Lighthouses, &c.
The object of this Invention is to join economy of light with splendour of effect. The means are the following:—
From the nature of reflecting curves, it follows that the smaller a luminous point is, the more perfectly will its emanations be reflected; for a focus is a point of the smallest magnitude, if, indeed, it has any dimensions. My idea, then, is to make a focus of a line of light very minute in it’s section, but as large, in it’s contents, as may be desired: thus securing a considerable fasces of luminous particles while using them in an economical manner. To this end (see [Plate 28], [figs. 3 and 4]), I form my reflecting surface of two distinct parts, having a section common to both, viz.—1st. a concave-parabolic-spindle, represented at A B C, as cut by a vertical plane passing through it’s centre; and 2ndly, a parabolical bason E D F G (represented in the same manner) surrounding the former, and so placed as that these surfaces have a common focus—namely, the circular line of which a b is the section; the line itself being shewn by an elevation passing behind the aforesaid spindle A B C. This linear focus, therefore, may be two or three feet in diameter; thus imitating the tenuity of a punctual focus, while emitting a large quantity of rays.
This Lamp, then, consists of an oil vessel, which is formed by the outside of the parabolical bowl before-mentioned, surrounded, in it’s turn, by the cylindrical surface P H, I Q, this vessel communicating with the wick-ring a N, b O, by a passage, H I, made as thin as possible, in order to leave the light at greater liberty to pass downward after reflection. (Where it is proper to add that the wick-ring is drawn too thick in the [figure].) Now, it is well known that all rays of light issuing from a point, and falling on the concave surface of paraboloid belonging to that point as a focus, are reflected from it in lines parallel to each other; and, therefore, a great part of the particles emanating from the linear (or circular) focus a b, and impinging on the surfaces F G A B, and B C D E, will be reflected perpendicularly downward, as at a, 1 3; b, 2 4, &c. and this being the case all round the common centre B, there will be formed a cylinder of light of the diameter H I, diminished only by the shadows of the wick-ring, the passage H N O I, and the pillar B L, when that is used, which is not indispensable.
If this cylinder of light strikes on the plane mirror K H, placed at an angle of 45° from their direction, these rays will be reflected horizontally, and, preserving their cylindrical form, may serve as a powerful beacon to the benighted mariner; the more useful, because susceptible of those temporary variations of direction and aspect, long since employed to distinguish one station from another.
But, if it were desired to illuminate a large space at sea, or elsewhere, the aforesaid cylinder of rays would be received on a conical surface K L M, which would give it the form of an immense sheet of light, of a thickness (allowing for aberration) equal to the height of P L M, of the same conical surface.