Faraday to William Crane Wilkins   14 December 1861

Royal Institution | 14 Decr. 1861

Dear Sir

Your question is not clearly put¹, but I suppose the following will include an answer to it. I speak however only of a first order light. A good standard lamp being in place & a reflector pannel placed before it all the ribs except the central one are shaded from the distant screen & the lamp is round or lowered until the maximum light of that rib falls upon the horizontal line on the screen then that rib is screened & the other ribs examined one by one to see that their maximum light falls on that part of that screen which is horizontal to the rib, the lamp retaining its first place. If the panel is properly constructed this will happen within very small limits. Then extinguishing the lamp the position of the burner below the central horizontal plane of the dioptric panel is ascertained & has been found by us to be 28mm below that plane. If such panels form part of an apparatus round 115 feet above the Sea then as the line proceeding to the sea horizon forms an angle of [blank in MS] with the true horizontal line the burner is raised that quantity corresponding to [blank in MS] mm in the axis of the apparatus.

With respect to the reflectors - the lamp & the dioptric panel are put into place - then a reflector pannel put into place with low ribs - then all but one rib shaded off - and then it adjusted until the maximum light falls on the screen in the horizontal position the rib is then finally fixed – each rib undergoes the same process[.] When all are fixed the lamp is put out, & the course of the rays for each rib traced back to the lamp & the place of their intersection noted. This for the upper reflectors appears to us to be 32mm above the burner and 20mm behind the axis - and for the lower reflectors 25mm above the burner & 40mm before the axis - As these points or foci are thus determined for the horizontal ray it is very easy to apply them to any other pannel & to any sea horizon ray. Thus if the lamp be round to set the refracting panels for a height of 115 feet, and a focimeter edge or gauge be applied at the unlighted lamp to indicate the focal point for the ribs of a reflector pannel and a small light be placed at the distant screen to represent the sea horizon for each particular reflecting rib, then it is very easy to adjust that rib in its place by looking over the gauge & through the rib at the distant horizon light and also to fix it there. Not only is this done in the manufactory but the whole apparatus is again examined in the lighthouse and if the ribs are not accurate they are readjusted by the sea horizon itself.

I do not know that I am quite the person to give these explanations. Perhaps they ought properly to come from the Trinity house[.] But I am sure of the feeling there that I do not hesitate to reply to you directly. My best remembrances to M. Sautter

Ever Very Truly Yours | M. Faraday

Mr Wilkins

I am at the seaside & have not my papers & tables here but the method is just as well indicated though the blank be not filled up MF