Faraday to Peter Henry Berthon   1 November 1862

Royal Institution | 1 Novr. 1862

Sir

At last I have obtained the precise data from Mr Wilkins¹ which his manufacturer (M Sautter) recommends for the foci &c of the Hunstanton 2nd order light; and am therefore enabled to reply to your letter of the 8th Octr²[.]

Because of the height of the light above the level of the sea (109 feet or 35 metres) M. Sautter recommends that the lamp be raised 2mm above its ordinary proper position i.e. that the burner be 26mm ( instead of 28mm) below the horizontal or optic plane passing through the center of the lens[.]

He recommends that the lens panels be inclined outwards at the top so as to make an angle with the upright of 10’. 27”, being the angle which a line to the sea horizon makes with the true horizontal plane.

The focus for the lens panels is to be 28mm above the burner in the axis - That for the upper reflectors is to be 36mm above the burner in the axis - and that for the lower reflectors is to be 6mm above the burner and 45mm aside before the axis[.]

Using the 2nd order lamp flame obtained at the Trinity house on the 8th Octr and sectional drawings of a second order apparatus, my conclusions for the foci (approximately) were for the lens panel 26mm above the burner, - for the upper reflectors 28mm above the burner & 5mm aside behind, and for the lower reflectors 12mm above the burner and 36mm aside in front. Now these are without experiment for I have no means of making experiments. Further they are results with one lamp flame only; - and further, when compared with the French data, especially in the mean lines for the upper & lower reflectors, they depart from them in only a small degree. I am therefore prepared to accept the French data in the present case.

I am also prepared to accept the elevation of the burner by 2mm.

The proposed inclination of the lens panels is perfectly accurate in principle - I am not prepared to say that any sensible good effect will be evident³.

Signed M. Faraday