George Biddell Airy to Faraday   6 April 1860

1860 April 6

My dear Sir

I have to thank you for the copies of your lighthouse lecture¹. (There is a graphic account of your Lecture in Moigno’s Cosmos², I suppose from one of the audience). I have little wherewith to make my return to you, and none to make an adequate one. But I send by Post a Notice about the approaching Total Eclipse which may interest you³. If you do not fear heat of climate, I would recommend you to go to see it.

This week, at the request of Admiral Hamilton, I have been to Birmingham, to look at the glass works in reference to the manufacture of lighthouse glass and to see one lighthouse which is now being packed up to go to Australia⁴. And I have been very much pleased with what I have seen, of the accuracy of the workmanship, which is quite sufficient for the use of a small light (as the galvanic spark) with the ordinary rules of direction of the beam of light, and to which they can give the modifications that may be required for any different sections of the beams of light.

But this struck me. We had the great lamp burning, not to its very full French height, but I believe as high as it is burnt in England. Upon trying the central (dioptric) division of the glasses by itself, and the upper and lower (catadioptric) divisions by themselves, the adjustment was evidently much in error. With a small galvanic spark, the error would have been much more conspicuous. This error was corrected by raising the lamp-stand 5/16 of an inch.

Now if a galvanic spark is used, can you be certain of its plane to a very small quantity (as 1/16 or 2/16) under the following conditions? - The two charcoal points are burning away, is it certain that they burn equally? The self-acting adjustment brings them together, does it move the two points equally? If one is equal and the other unequal the vertical position of the spark will vary.

If the galvanic spark shall be introduced, for which I think you have shewn strong reason, it will be of great importance for economy of light to attend to the vertical of adjustments. The horizontal spread in the revolving lights will require special optical arrangements.

I am, my dear Sir, Yours very truly | G.B. Airy

Professor Faraday