Faraday report to Trinity House   3 December 1860

Report of experiments at Birmingham on the focal points of the lenticular panel of a fixed first order lighthouse apparatus.

3 Decr. 1860 | Royal Institution

I have been engaged at Birmingham for three days (26, 27, 28 November) with Mr. James Chance in ascertaining for each particular rib of such a panel as that referred to above the best point or points in relation to the burner from which the rays should issue that the maximum illumination might be at the horizon; and also testing by practice the method proposed by Mr. Chance of adjusting the ribs to each other & to the lamp, so that they should coincide in their action¹[.]

A fixed-light lenticular panel consists of a chief broad middle band, and of other bands or ribs above & below it, forming two groups, generally nine in each. These when associated in the panel frame have according to the accepted rule their common focus at 28mm above the burner of the lamp[.]

The relation of the flame & burner to the upper and lower parts of the lenticular panel, is the same in kind though not in degree as their relation to the upper & lower reflectors: thus the opaque burner which hides much of the flame from the lower reflectors also hides flame from the lower part of the lenticular panel though not in the same degree[.] But as the lenticular panel is a very chief part of the apparatus any desirable correction which can be made amongst its parts, if possible, becomes of importance.

Using a flame such as that represented in the Report of the 19th Octr. 1860², in conjunction with a panel of the kind described, and casting the light passing through each rib separately in a perfectly horizontal direction, we found that the best focal point for the middle or chief rib, was 20mm above the burner at the axis;- that the upper ribs though varying one from another, might have the same points of 20mm taken for their average or common focus;- and that the lower ribs required focal points in the axis varying from about 18mm to 30mm above the burner, all of which might be referred to a common focus 11mm up and 36mm aside towards the panel.

Supposing that these numbers (or any other) were determined upon then the possibility of adjusting the parts of the panel to each other came to be considered; without which possibility it would not be right for the authorities to require that a finished panel should be subject to examination by the focimeter in relation to such given points. The ribs of a lenticular panel cannot be adjusted to each other by any rotation of them on a horizontal axis, as is the case with the ribs of a reflector panel; but only by elevation or depression in respect of each other:- and now Mr Chance proceeded to show me how, by ascertaining the best focal point for each rib and their relation to the focal point of the great central rib, he ascertained how much they were in error, and then what proportion of glass would require to be removed from the broad bearing surface of this or that rib to bring the whole into nearest approximation to the desired position. This he carried into effect with the panel which we had had under examination, and which had been constructed in the ordinary way, & without any particular view to such a correction; and the consequence was, that a panel was produced, which when set up with the focimeter upon the burner at the numbers given above, & a small flame upon the distant (107 feet) dead level for each rib, gave a perfect practical result:- the space between the green & red light was the part of the flame observed, and the error for any rib was not more than 1mm, except in two cases out of nineteen in which it did not exceed 2mm. When the great lamp was lighted, the effect was in accordance with the expected result. The coincidence of all the rays in one common maximum could only be observed at a great distance i.e. at the dead level horizon; but each rib could be examined for itself and for the dead level of that rib.

It must be thoroughly understood that the focal numbers have relation to the flame of the great lamp. As before stated, the higher & more powerful the flame the greater height should the focal distances be above the burner, but even with a very high flame we do not find that the focal point of the middle belt & upper ribs can be raised higher than 23mm or 24mm above the burner without sending the brightest light to the sky. The character and size of the flame differ very much at times; and even with the same flame persons differ very much in their estimate of its magnitude & the place of its brightest part. We have taken a flame corresponding, as far as I can judge, with that figured in the report of the 19th October 1860. It is easy to obtain a higher flame by close attention & for a short time; but I do not think that a higher one is often sustained in lighthouses[.]

In respect of the Small’s light, therefore, I recommend a focal point for the central & upper ribs of the lenticular panel of 20 (or 21) mm above the burner; and for the lower ribs, a point 11mm up and 36mm aside in relation to the dead level:- and a further correction of 3mm for the dip to the sea horizon.

The final examination of an optic apparatus for lighthouses can only be carried on with the degree of accuracy which I have described at the manufactory; or at a place arranged with all appliances for the purpose; or at the lighthouse (when in place) when there is a good clear sea horizon. In the case of the “Smalls” no difficulty will occur; since the apparatus is already in the hands of Messrs Chance.

Perhaps it may be agreeable to the Trinity house to be informed that the changes proposed now & formerly are all in accordance with observations made by the Astronomer Royal at Messrs Chances in the beginning of the year and which he communicated to me personally in April last³[.]

M. Faraday

3 Decr. 1860⁴.