Faraday to James Timmins Chance   25 August 1860

[Royal Institution embossed letterhead] | 25 Aug 1860

My dear Sir

I have just time to acknowledge your letter¹ & thank you. I give you much trouble.

When I came home and compared your real experimental foci with those I had obtained by a first rough tentative proofs (on paper only) I was gratified to find that I was not far from you for I accept your determinations as real practical results. I had made a point 21mm above the burner and 50mm from the central vertical towards the side of the lower reflectors a common focus for the illuminating rays going to the middle of the face of each reflector. In like manner I had adopted for a first approximation a point 26mm above the burner and 31mm from the vertical axis of the system on the side opposite to the upper reflectors as a common focus for them. This would give on the vertical axis foci varying from 45 to 125mm - instead of the constant French focus of 38mm[.]

I had taken a point higher & nearer to the vertical axis but think that experiment with such a flame as one has a right to expect will give the former numbers or something near to them. I wonder what you will get by experiment[.] How I should like to come & experiment with you[.]

With sincere respects to Mrs. Chance - and what I forgot very many thanks for her kind intentions regard the Infant Orphan Asylum & little Leighton²[.]

Believe me to be | Every Truly Yours | M. Faraday

James Chance Esq | &c &c &c