Faraday to Peter Henry Berthon   14 November 1861

Royal Institution, 14 November 1861.

Sir,

I had the honour of accompanying the Deputy Master¹ and Captains Drew and Were, on board the yacht, to sea, on the night of the 30th October, for the purpose of observing the lime light at the Upper South Foreland, and comparing it with the reflector lights at the Lower South Foreland Lighthouse. The Deputy Master has requested me to state my opinion², and though I have no pretension to compare with a seaman’s judgment, I obey.

We went on board about six o’clock, steamed out, and up and down channel, 10 or 12 miles or more off and across, for about three hours. Comparing the two lights together, of the two the lime was the best, and often much so, especially when near shore; further off, the difference was less, probably for want of adjustment of the lime light. On different bearings both lights vary, and sometimes the low light was even better than the upper light. The variation of the reflector depended chiefly upon the difference of the light in and between the successive reflectors, and perhaps, at times, of the window-bars. The variation with the lime light depended upon the different degree of light at different jets (which was considerable), and the intervention of the window-bars or astragals before the front of any given jet light. The lime light was not so superior 10 or 12 miles off as when nearer. The light is probably directed too much downwards. The reflection of the sea was strongest for the lime light, and especially at the near distances. The lime light was generally whiter than the lamp light, but occasionally they were alike. When about equidistant from Grisnez and the South Foreland, the brightest light at Grisnez was not greater than the constant light of the Upper South Foreland or lime light; but of course the weather or atmosphere on the French coast was unknown.

The North Foreland Light was seen 16 or 17 miles off, a very fine light. The effect of the adjustment by Mr. Chance, and of the good lamp, was very striking.

On the whole, the lime light was not so superior at sea as I expected. No doubt it was generally much superior to the oil lamp and reflectors in the low lighthouse, though on certain bearings only equal to them. It must be remembered, as I have said on a former occasion, that the optic apparatus is not in adjustment or it; and that will have something to do with the result. I consider it as (in power) much beneath the electric light, which was formerly shown from this same lighthouse³.

I have, &c., | (signed) M. Faraday

P.H. Berthon, Esq., &c. &c.