Faraday to Peter Henry Berthon   20 February 1863

Royal Institution, 20 February 1863.

Sir,

On Tuesday last (the 17th) I proceeded to the Dungeness Lighthouse, to examine the new optic apparatus (which is the upper of the two) provided for the Magneto-electric Light. The examination was in every point satisfactory. The apparatus is of the same general size as the former. It collects on the glass all the chief rays, letting only a little escape at the bottom and behind. The grinding and adjustment of the glass appear to be excellent, and its quality very good. The outgoings of the rays at night-time appeared to be right.

On the 18th continued the examination, using the sea horizon; and arranged for observations to-night from the Varne light, for the purpose of comparing the ordinary lamps and reflectors with the new apparatus, as on a former occasion, and also comparing the new and old apparatus together.

At night, went out from Dover to the Varne light; fine night. The reflectors (five in number) towards the Varne, could be seen by themselves, and were about as the Upper South Foreland. When changed for the Electric Light the latter was overpowering; overpowering in this respect, that when the latter was shown alone, the addition of the reflectors made no sensible difference; and when both the Electric Light and the reflectors were shown together, the shutting off of the latter caused no apparent change.

Then the reflectors were taken down, and the old and new optic apparatus compared with each other, the Electric Light remaining the same. The new one came out greatly superior. Whenever the change was made from one to the other, it was seen in a moment, and there was no occasion to refer to the time table to know which apparatus was in use. Thus the expectations raised by yesterday’s examination were fully confirmed.

Steamed in towards the Dungeness light, and came upon the distance at which on the former occasion, it was thought that the old apparatus concentrated its rays (9½ miles). Here, as elsewhere, the new apparatus sustained its superiority over the old one.

Endeavouring to compare the maximum of the Grisnez with the constant new Dungeness light (as before) by the naked eye, and by the penetrating power of a wedge-shaped dark glass, sometimes one was superior (or I thought so) and sometimes the other. My conclusion is, that they are as nearly equal, by ordinary observation, as they may be. The general appearances seemed to show that there were local hazes about in various places and directions, forming and changing from time to time.

Returned with the Deputy Master¹ to Dover, and came to town the next day.

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

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