Faraday report to Trinity House   14 September 1860

Report on experiments at Messrs. Chance’s works and at Whitby in relation to focal points.

I have been to Birmingham and worked for two days, with Mr. James Chance, upon the determination of the best focal points for the upper reflecting prisms. I still see reason to expect much improvement by the change referred to in my report of the 1st instant¹, and now propose to carry out that change, experimentally, at Whitby.

For this purpose, I propose (as before), that the North Whitby light be left untouched, for a standard. That Mr Chance provide for the South light a good overflow lamp, with a continuous chimney 6 feet in length from the bottom of the glass, and adjust it so, that a ray from the sea horizon passing through the middle of the Lenticular zone, shall intersect the axis of the lamp at a point 28mm above the burner. That the lower reflecting prisms of all the Octants save the third counting from the north, be adjusted to the sea horizon, from a focus 20mm above the burner, and 50mm from the lamp axis towards the reflectors. That the upper reflecting prisms (save those of octant No. 3) be adjusted to the sea horizon, but from different foci:- the north pannel or No. 1 to a focus 20mm above the burner and 30mm from the lamp axis on the side from the reflecting prisms:- pannel No. 2 by a focus 28m.m above the burner and 30mm aside:- the South pannel or No. 4 by a focus 28mm above and 40mm aside. The Octant No. 3 to have both the upper & lower prisms adjusted by the French foci and not to the Sea horizon but to the true horizon.

I then propose to go to sea at night time, and examine the effect of these different adjustments at distances up to 18 or 20 miles; moving on some such course as that, a plan of which, is herewith sent (marked P), that the effect of each adjustment may be observed and compared with the North Light:- and, I propose that, at the extreme distance, the lenticular band shall be screened off, so that in returning, the effect of the reflecting prisms only shall be seen & compared; the Northern light still remaining unchanged.

This sea trip will probably decide the best arrangements:- but if cause appears for trial of any other foci, - or any other arrangement, such arrangements can be made in a day or two, and a second night-examination at sea be made.

The expence incurred by the experiments at Birmingham, by those now recommended at Whitby, and for apparatus I have had occasion to order, will probably not exceed 300 pounds²[.]

M. Faraday

Royal Institution | 14 September 1860.