Faraday to Frederick Madan   12 November 1852

R Inst | 12 Novr. 1852

My dear Sir

You know far more about lenses practically than I do & I do not know that you are under any mistake¹. Practically I know nothing of them and cannot have the opportunity, and it strikes me as very serious that there have not been practical comparisons of the lenses (as well as of the refractors) with an Argand & reflector at Purfleet & Blackwall[.] It would be very easy so to shade the parts of a lens as to make a comparison with a reflector that should be very useful. The axial part of the beams could be compared and the divergence might be laid hold of by some contrivance so as to make the comparison more complete[.]

I referred to a divergence of 15˚ because I understand it is essential that the character of the Caskets lights should not be altered[.] If you were to contract a divergence of 15˚ into one of 5˚ or 6˚ and so make the duration of the light only about one third what it was before I concluded that could not be allowed[.]

Ever Truly Yours | M. Faraday

Captn Madan | &c &c