^{page 35 note *} Supplément à l' Histoire Naturelle, tom. ii. 12^mo Paris 1774.

^{page 37 note *} “The burning focus lies a little beyond the red rays, and is therefore at a greater distance from the lens than the luminous focus.”

^{page 41 note *} The lenses in this lighthouse, which are two in number, are twenty-two inches in diameter.

^{page 42 note *} This lamp has been brought to a high degree of perfection by MM. Arago and Fresnel, and is a most valuable addition to the apparatus for lighthouses.

^{page 42 note †} See Edin. Phil. Journ. vol. viii. p. 165.

^{page 45 note *} “The reflectors,” says Mr Stevenson, “consist of a circular sheet of copper, measuring, when, flat 26¼ inches in diameter; weighing 11½ . on an average, and plated with silver in the proportion of 6 oz. to each pound avoirdupois of copper. These plates are formed into the parabolic curve by a very nice process of hammering, and afterwards set into a bezil or ring of brass.”—Account of the Bell Rock Lighthouse, p. 527.

^{page 46 note *} See Art. Optics, Edinburgh Encyclopædia, vol. xv. p. 607.; and Biot's, Traité de Physique, tom. iv. p. 579Google Scholar.

^{page 50 note *} See Edinburgh Encyclopædia, Art. Optics, vol. xv. p. 67.

^{page 50 note †} A burner with two concentric wicks should be immediately introduced into the lamps now in use.

^{page 51 note *} That the reflectors for lighthouses are considered by competent judges to require improvement, appears from the following passages: “It is greatly to be desired,” says the Editor of the Bibliotheque Universelle for July 1826, “that the perfection to which optical instruments have been brought, should be extended to that branch of the science which has for its object the illumination of lighthouses.”

“From certain experiments now in progress,” says Mr Stevenson, “the writer is in expectation that considerable improvements may be introduced in the construction of reflectors; and that additional modes of distinguishing the lighthouses on the coast will be obtained.”—Account of the Bell Rock Lighthouse, p. 527.

^{page 52 note *} A central lens of this size may be easily executed, in flint-glass, free from any considerable imperfections, for the late M. Fraunhofer. undertook to execute a flint lens for achromatic telescopes, eighteen inches in diameter; and M. Guinand actually made one of that size.

^{page 57 note *} By the interposition of the second lens AB, a much larger cone of rays is thrown into the main beam by the lens LL than could have been done without AB.

^{page 60 note *} In his Account of the Bell Rock Lighthouse, p. 322., Mr Stevenson adds, “that, after the most full and satisfactory trials, the red colour was found to be the only one applicable to this purpose. In tolerably clear weather, the light of one reflector tinged red was easily distinguishable, at the distance of eight or nine miles; while the other colours rendered the light opaque, being hardly distinguishable to the naked eye at more than two or three miles.”

^{page 64 note *} In order to render available the reflectors of the old system, the following combinations may be adopted with advantage in many cases.

As the back part of the reflector is almost useless, an aperture two or three inches in diameter may be cut away at D, Fig. 5., so as to give free passage to the cone of rays FAB, which, falling upon the lens AB, will be reflected into a parallel beam ARBR.

Two reflectors CDEE', C'D'EE' may be coupled together, as in Fig. 6., so that the lamp F may be exactly in the focus of each, and in this manner we shall have two beams of light in place of one.

Or we may give additional power to the reflector, as in Fig. 7. by using another lamp F, and surrounding the reflector with the external zones of a lens AB, in whose focus the lamp F is placed. The column of light CDRR, thrown out by the reflector, will be widened on all sides by a hollow cylinder of light, whose section is ACRR, DBRR.

^{page 65 note *} This advantage is strikingly pointed out by the following fact stated by Mr Stevenson: “After having corresponded with all parts of the kingdom in endeavouring to procure red glass of the finest quality, by having it coloured in the furnace, it was mortifying to find, that its manufacture was wholly impracticable, excepting in the production of small pieces not more than three or four square inches, similar to those in the compartments of cathedral windows, which, in the process of shading a reflector, must have induced a number of minute divisions, and necessarily obstructed much of the light. The writer at length resolved on confining his attention to plates of crown-glass stained by repeated application of the litharge of gold, laid on after the manner of gum or paint, which was afterwards subjected to a strong heat in a muffled furnace of a peculiar construction, forming altogether a very nice and difficult process. ***** Although the effect produced in this way cannot be so perfect as if the glass were uniformly coloured in the pot, yet, when applied to the purposes of a distinguishing light, its effects are highly characteristic and beautiful.”—Account of the Bell Rock Lighthouse, p. 392.

^{page 67 note *} If gas were used, we might, on such occasions, employ a burner ten inches in diameter, and having many concentric flames.

^{page 68 note *} See Edinburgh Journal of Science, No. X. p. 139.

^{page 67 note †} Since writing the above, I have learned that gas has been used in one or more lighthouses.

^{page 69 note *} The economy in oil, in wicks and in lamps, must be very considerable, and, were it necessary, might be easily valued. In lighthouses which are near towns where gas is compressed, and to which it could be sent by sea-carriage, portable gas might be introduced with the most obvious advantage.

^{page 70 note *} Edinburgh Encyclopædia, Art. Lighthouse, Vol. XIII. p. 10.