Faraday to Chance Brothers   6 October 1859

Royal Institution | 6 Octr. 1859

Gentlemen

I have received from the Trinity House letters from you, and a drawing, respecting a second order revolving light¹; they refer to a proposal by you of certain modifications &c, mentioned particularly in your letter of the 29th September last. I do not feel as if I saw all your reasons for the proposed modifications & therefore write to ask you. The first modification consists in placing 16 upper & 6 lower catadioptric holophotal prisms in the space occupied by 12 upper & 5 lower prisms in the old system. What is the improvement expected here? Is it in the greater accuracy of workmanship?- or the greater strength of construction?- or the greater proportion of rays gathered in?- or in any other effect?-

Then the second modification seems to be to construct the apparatus with 12 annular half lenses & 12 pannels of prisms - instead of 6 or 8.- I am not sure that I understand what a half lens means; but suppose it means a narrow one - one with the two sides cut away- You seem to consider 12 segments the best number for a light revolving at half minute intervals but as I understand you only in relation to speed of motion. An apparatus with six pannels would have to revolve once in three minutes for such intervals:- whilst with the 12 pannels it would revolve once in six minutes. Does the expected improvement consist in this difference in the velocity of motion? or is there any other effect that is contemplated, because of the difference in the size of the lenses?

Is there any other modification of effect beyond these two which I have to consider before I reply to the Trinity Board?

I am Gentlemen | Your Very faithful Servant | M. Faraday

Messrs Chance Brothers & Co | &c &c &c