John Frederick William Herschel to Faraday   26 October 1828¹

Dear Sir

The annexed Borate of Lead was made in the year 1821 and as you see is no way tarnished. It was made by prescription & fused in a Wedgewood basis - yet it is still yellow.

The other specimen is a trisilicate and is nearly saturated - I should like much to have a pounding up of it and refusion in a clay crucible, as it seems only to want a longer heat and more perfect union of the undissolved skeletons of Silex at the top with the rather excessive proportion of Lithage below to make a good glass. It+ does not seem at all to have attacked the Crucible.

This has a tarnish on its fracture though made only about 2 years.

I remain dear Sir | Yours very truly | J.F.W. Herschel

P.S. On considering the matter of parallel strata in glass, I perceive for the first time that they may be dreadfully injurious. For let PQRS be a concave flint lens worked from a disc disposed in parallel strata 1 2 3 4 5 6[.] Then all the rays Aa Bb and Ee Ff which fall on that ring of the polished surface whose sections are ab, ef, where the Stratum 1 cross out will have a focus determined solely by the refractive power of the material 1. Those which fall on the [word illegible] of 2 will have their focus determined by the refractive power of 2 and those of cd by 3. Thus these several cones will not have a common vertex, & an aberration of a kind analogous to, but worse (because more uncommandable) than the spherical will arise.

To obviate this the disc should be softened and pressed in a mould which will divide it into a series of concave lenses thus, and all the rays will enter at a median of one refractive power and all will emerge at one of one other.

PPS. | Query then are not many bad object glasses faulty from the crown - not the flint lens.

Query also whether advantage might not be taken of this in the making of large burning lenses.

Address: M. Faraday Esq | R. Institution | Albemarle Street