Faraday to Peter Henry Berthon   21 June 1858

Royal Institution 21 June 1858

My dear Sir

I have examined the block of glass sent to me with your letter of the 18th¹ and compared it in respect of colour with numerous other specimens of plate glass. I find it to come out exceedingly well. It will not compare with flint glass nor should that be expected. When a thick block like that you sent me is looked at in the ordinary way much of the light which traverses its length or breadth comes to the eye. These parts are here 5 & 3 inches & the effect is as if glass 5 or 3 inches in thickness were looked through[.] I have in the present case blocked that out & the Deputy master² will now see the colour belonging to a thickness of 7/8 of an inch³[.]

I am My dear Sir | Yours Very Truly | M. Faraday

P.H. Berthon Esqr | &c &c &c