James Timmins Chance to Faraday   24 April 1860

Hamstead, | Birmingham, | 24 April / 60

My dear Sir,

I am obliged by your letter of yesterday-¹. When Profsr. Airy first inspected the apparatus, the lamp was placed with the top of its burner 28mm below the focal plane.

With the height of flame then attained, 20mm was found to be about the best distance of the top of the burner from the focal plane.- This is the chief point to which I imagine Profsr. Airy to refer - viz. the best distance of the burner below the focus.-

I believe that 20mm is not unusually adopted by the Trinity House - or about that distance.

At all events the position of the focal plane in the flame is still an undecided one - and a very important one.

On a subsequent examination of the apparatus which Profsr. Airy saw, I determined still to adhere to 28mm:- but I made a slight alteration in the position of the lower prisms in their panels.

I have never seen a better light than the one alluded to:- I should like you greatly to have inspected it.

I am glad to hear of your success with the electric light - for fixed lights².

In a sixth order Light (150mm say) a height of flame equal to 1 mm (say) = 1/25th of an inch, ought (even supposing parallel emergence) to spread over 20 miles from the horizon inwards (say) at an elevation of 400 ft:-

The remaining distance to the shore can be provided for by the bottom prisms.

There will be an end I suppose of large apparatus.

Yours truly | J.T. Chance