John Frederick William Herschel to Faraday   6 June 1828

Dear Sir

Enclosed is a little prism cut from a specimen of glass I brought to the R Institution yesterday of 2 borax - 2 Fluor - and 1 sand[.] The polish on its faces is so irregular that it gives but a very bad image - but I should judge it by a rough comparative trial I made of it with a little water prism having nearly the same deviation to possess a dispersive power considerably below water & therefore below crown glass, which is what I expected and hoped.

In the table of Dispersive powers we have as follows

The borax probably means crystallised borax. That of the fused will therefore probably be lower than 30 but taking 30 we might expect to have

for the dispersive index

(1000p) of the glass[.]

Now 48 : 35 :: 35 : 25.5 so that, on this estimate which is probably above the truth the glass ought to be as far below crown glass as that does below flint, and therefore the composition will be equally easy with the fluorine glass as with flint, and (from the much less quantity of colour to be corrected) probably much more perfect.

Yours truly | J.F.W. Herschel

Kensington June 6. 1828.

Address: M. Faraday | Royal Institution | Albemarle Street