Faraday to Peter Henry Berthon   15 March 1861

Royal Institution | 15th March 1861

Sir

I proceed to reply to your letter of the 13th instant¹ which was accompanied by a report from the MM. Stevenson. This report relates the results of experiments made to determine whether the maximum light produced by a four wicked Fresnel lamp placed behind a first class cylindric refractor was thrown above or below the horizontal axis when the upper edge of the lamp burner was 28mm below that axis. Four sets of trials were made & in all of them the result was that the chief beam was thrown below that line the average depression being 30’.

These experiments were made in consequence of like investigations entered into at Birmingham by Mr. Chance at which Mr Thomas Stevenson was present the apparatus being alike i.e a four wicked Fresnel lamp with a first class cylindric reflector having been used: a different result was obtained inasmuch as according to the report above “the larger portion of light” was thrown above the axial line[.]

Though not present with Mr. T. Stevenson at Birmingham I have no doubt I have seen there at Mr Chances works like experiments to those he refers to. They are described in my report to the Trinity house of the date of 3 Decr. 1860² in the sixth paragraph. I give my testimony as there expressed. The results at Edinburgh & at Birmingham differ but I cannot state why they differ not having seen those at Edinburgh. I have in the paragraph referred to especially associated the Birmingham results with the character of the lamp flame and as far as I am acquainted with lamp flames I have never seen one which was likely to be sustained by the keepers during the ordinary practice of a lighthouse at such a height & in such a state as would give the Edinburgh result: but I have never seen the Scotch lamps burning in their ordinary or even in any other condition[.]

Mr. T. Stevenson who has seen both the Birmingham & the Edinburgh experiments is in a much better condition to judge of the cause of the difference than I am. At the close of the report (page 5) he seems to refer it to the difference of the flames.

In relation to the flames really obtained in lighthouses and the direction of the chief rays proceeding from them I may I think refer to the observations & results of the Royal Commission on lights, the members of which with the astronomer Royal having personally examined lighthouses in England Scotland and France found as I understood, with but one exception & that in France, that the chief rays proceeded upwards to the sky more or less, a natural & necessary consequence of the flames being too low as regarded the optic apparatus. These practical observations were in fact the leading cause of the focal investigations made at the Glass works at Birmingham[.]

Such a difference as that existing between the Birmingham & Edinburgh observations might be caused by the persons at either place having erred in the levelling & other arrangements of the apparatus, but I do not see any reason to suppose such a cause - The levels & at Birmingham were gone over several times & I went over them once myself[.]

I have the honour to be | Sir | Your Very Obedient Servant | M. Faraday

P.H. Berthon Esqr | &c &c &c