Faraday to John Francis Campbell   26 March 1860

To J.F. Campbell Esqr | &c &c &c | Secretary. Royal Commission

answers to second set of questions ¹

I presume that in Gt Britain as abroad there is always a competent authority who considers & decides the circumstances of every particular lighthouse & gives instructions to the competing manufacturers[.] I think a manufacturer should not on his own judgment interfere with the judgment of such an authority

He may know the height of the lanthorn above the sea but ought not to vary his angles for any variation in that respect. The variation of the angles of the glass is rarely if ever the kind of change required. The authorities are conscious of the need of making the beam drop if necessary & always have the power of deciding it. In respect of the horizontal arc also the authorities always I believe give the number of degrees and the manufacturer should not use his judgment in the matter[.]

2 The manufacturer does not need this information to guide him in the grinding & fitting of his lenses prisms &c. He should receive a specification for an apparatus of well known construction to perform a certain well known optical duty and if when it is examined it does not perform that duty it is rejected.

3. There is not the slightest occasion for a table relating to the horizontal arc. The instruction is in any case for a certain number of degrees; with frame bars &c in certain plans and the work simply to be done. As to elevation the depression of the ray necessary on that account is in the power of the authorities being either ordered especially or governed by adjustment of the lamp and it should not be interfered with by the manufacturer[.]

4 I have no duty in this respect only that of examination nevertheless as far as I know by the results the specification & instruction given by the authorities are sufficient for the purpose. Perfect numerical exactness in the angles &c cannot properly be directed or required: for the maker has rightly a power of improvement over his glass & if his glass vary the angles must vary also. The specification should tell every thing that is missing for the full information of the makers. The makers have their work subjected to a very close optical scrutiny, and it is probable that any attempt to refine further in their department might lead to the removal of responsibility from the parties who ought to bear it.

26 Mar 1860 | M. Faraday