Faraday to William Pigott   22 July 1864

Royal Institution | 22 July 1864

To the Deputy Master | Trinity House | &c &c &c

Sir

The optical apparatus which I had the honour of examining in company with Yourself & others at Birmingham and which may be called Chance’s Gibraltar apparatus was of the first order and intended as a fixed light to illuminate about 290° of the horizon by the usual central four wicked lamp aided by the usual refractive & reflective prisms of Fresnel but some especial results were desired and some especial apparatus was devoted to the purpose of obtaining them[.]

At about 5 miles from the Gibraltar station for the light and I think in a Westerly direction is a danger point called the pearl rock which with the space between it & the coast comprizes 25° of the horizon is to be marked by a red light whilst the 210° of open sea before it & the 55° to the W. & N.W of it within the bay are to be marked with white light. The 25° corresponding to the danger were to have the light sent to them reddened by the use of Red glass applied in the front of the optic apparatus for that purpose but as that would weaken the warning light where it rather required strengthening i.e. over the 25° between the pearl rock & the coast, use was made of the 28° of light beyond the 290° not required in the upper part of the bay. For this purpose six reflecting vertical prisms each in three lengths was fixed up outside of the part of the optic apparatus & receiving the light which was issuing thence in the horizontal plane turned it to one side & threw it over the 25° between the pearl rock & the coast[.] Five of the vertical prisms acting as plane reflectors did this without otherwise altering the course of the rays and the illumination would therefore be greatly strengthened though nearly the whole of the 25° but as the pearl rock required an especial amount of illumination the sixth prism was so formed as to act as a concave reflector in the horizontal plane & adjusted so as to throw its whole flash on to the pearl rock. The whole of this light was to be reddened by the interposition of red glass.

This apparatus was examined both by day light & at night in the latter case especially with reference to the direction in which this light was thrown. It was found to be admirably accurate in the action of its parts. The light proceeded in the direction in which it had been ordered to go, and though the final adjustment of the sixth or flash or pearl prism was left to be made at Gibraltar yet I have no doubt it will be with the effect expected[.]

In all other respects the apparatus was as desired. The glass was good in respect of striae bubbles colour adjustment & workmanship:- it claims & has my full approbation¹

I have the honor to be | Sir | Your Very Obedient humble Servant | M. Faraday