Faraday to William Crane Wilkins   20 October 1862

Royal Institution | 20 October 1862

My dear Sir

In my note to you of the 13th instant¹ I asked for the proposed foci in heights above the burner for of course it is by such heights that the verification would take place in the light house. The expression in heights above the horizontal plane would give by a little calculation the results wanted but the relation of the horizontal plane to the burner is somewhat doubtful. You will see that by the notes I send you marked I and II which please return to me:- for the burner according to the drawing and note I is to be <²>26mm below the horizontal focal plane:- but according to note II <(2)> when the light is elevated 35 meters the burner is still to be 26mm below that plane and yet at <(3)> it is said that the burner should be 2mm higher for the latter case than the former.

I should understand the rest of the note II (4.5.6.) which refer the elevations to the horizontal plane were it not that the relations of that plane to the burner appear to me to be uncertain by 2mm[.] Perhaps I have made some mistake but I cannot find it out:- <(4)> seems to imply that the burner is to be raised 2mm.

I am | Your Very Obedient Servant | M. Faraday

W. Wilkins Esqr