Thomas Stevenson to Faraday   26 October 1860

Edinburgh | 84 George Street | Oct 26 1860

My Dear Sir

I beg to acknowledge receipt of your letter of the 24th [sic]¹.

I send you in case it may be useful some photometric experiments lately made by my brother² on the vertical divergence of an annular lens with a flame of our full standard height and volume from which you will observe that the most minute beam dips downwards a little. You will perhaps remember that I shewed a tracing of it at Whitby. The observations were made at the sloping face of Salisbury Crags and the light was in a temporary lantern placed at the South & on the back of Canongate. We would also have tried the prisms but had none fitted up at the time. The levels of each station was [sic] carefully ascertained with the spirit level and from these angular values marked on the vertical line were computed.

Do not put yourself to the trouble of acknowledging receipt of this but if any further explanation be required I need not say how glad I shall be to afford it.

I remain | My dear Sir | Yours very truly | Thomas Stevenson

Professor Faraday | &c &c

Endorsed by Faraday: What was the height of the focal plane above the burner? | What is the standard of intensity or how was it ascertained? | What is the cause of diffusing at the focal plane?