Faraday to William Pigott   21 April 1864

Green | Hampton Court | 21 April 1864

The Deputy Master | Trinity House

My dear Sir.

I have your note of the 19th here¹

The experiments made at the Trinity House on shadows cast by upright standards & astragals give me no occasion to change what I said at the end of a letter to Mr. Berthon (8 March²) or that said in another to Captn. Arrow (14 March³).

In the experiments of the 14th instant, shadows of the least intensity were cast when the uprights of the lanthorn & of the optical apparatus were quite clear of each other but in this state the greatest whole amount of shadow in the 360° was sent seaward:-

When the uprights were so placed that they did not coincide but stood side by side in the direction of the outgoing ray then the joint shadow of the two appeared at times to be more than the sum of the two and would be disadvantageous for its particular direction; but it is different for different distances and would not be the same for a distance at sea as in a limited room:- There is less absolute shadow in such a case over the 360° but the conjunction is I think to be avoided as bad for the particular direction.

When the uprights of the lanthorn & the optic apparatus coincide accurately, there was the best amount of absolute shadow i.e the best amount of loss of light in the 360° of any of the cases: and the shadow of the two was so little more than the shadow of the one that I think such an arrangement is to be favored generally[.]

Whether the astragals &c were upright or vertical provided their inclination caused them to cast shadows parallel to each other the effect was the same[.]

A full sized model of part of a lanthorn on the construction of Mr. Douglas was placed before the lamp & optic apparatus[.] It cast no sensible form of shadow on the screen[.] In fact the whole amount of shadow was a minimum & it was uniformly (or nearly so) diffused over the illuminated interval. I conclude that it would be nearly a matter of indifference as regards shadow where the uprights of the optical apparatus were placed in relation to such a lanthorn⁴.

I am | My dear Sir | Your faithful humble Servant | M. Faraday