Faraday to Peter Henry Berthon   1 September 1860¹

Royal Institution | 1 Septr 1860.

Report on Focal points

In reference to the examination proposed at the end of my report of the 16 August 1860² of the Focal points heretofore adopted in France and also in this country I have to state that from considerations founded upon the size of a good lamp flame I came to the conclusion that the focal point for the lenticular or dioptric part was right or very nearly so but that those adopted for the upper & lower reflectors was much too low. Those for the lower reflectors may be represented by a common intersecting point 10mm above the burner & 48mm from the axis on the side towards the reflectors. My first judgment was to alter this point by raising it so that it should be 20mm above the burner & 50mm from the axis. Mr James Chance invited me to see two fine first class lights for Russia one of which being a fixed light he had of his own judgment & experience adjusted to foci different to the above. I found the effect of this alteration to be very excellent and I found moreover that his numbers & mine were so near to each other as to render them in effect coincident³[.] I consider the result therefore a full practical confirmation of the numbers & I do not hesitate to recommend this alteration for the adoption of the Trinity house[.]

With regard to the upper reflectors my preliminary judgment was for a far greater change. The French authorities (which we in England have thus far followed) is for a common focus for all the upper reflectors placed in the axis of the flame, & raised 38mm above the burner. I think the common focus ought to be 27mm above the burner and 32mm from the axis on the side from the reflectors. I have endeavoured experimentally with a good lamp to establish or correct this estimate and my results gave a point 30mm above the burner & 32mm aside from the axis as an excellent place: but I desire to carry out this change upon a whole pannel of upper reflectors before I recommend it finally to the Trinity Board⁴.

M. Faraday

Endorsed by Faraday: (Lamp must have been very good. Wilkins reports that burnt 7 pints? of oil in 4 hours⁵).