Faraday to Peter Henry Berthon   20 July 1858

Eastbourne. 20 July 1858 - Sir, On the Receipt of your letter of the 15th¹ at Eastbourne I instantly went up to London and finding M. Sautter the constructor of the Bishops Rock apparatus arranged to meet with him at the Trinity House and explain to him my mode of examination. Unfortunately there was no part of the Bishops rock apparatus that had not been sent off but we placed up & examined an old French pannel of excellent character and also a new one of M. Sautter which were quite sufficient to illustrate all the points we had in view.

M. Sautter has since that written to me in reference to my Report to you and I enclose his letter marked S.A. 19 July 1858² and marked also in the margin for reference. You will see that he admits the correctness of my criticisms (D) but adds observations which should influence their effect on the final judgement of the Trinity House. In my reports I have never presumed to express an opinion as to the acceptance or refusal of an apparatus. My object has been to state how near to or far from perfection any apparatus was, not that perfection is ever attainable but to mark the relative position in which any particular apparatus may stand - to prevent retrogression - and to point out what may yet be wanting. To do this seems to me the more important because if a concentrated light, intense in effect, but small in dimension, as the Electric light or the lime light ever becomes available, it will require a perfection of work & quality which may easily be dispensed with as at present when a large Fresnel flame is used.

I will now refer to the three points of 1 Colour, 2 Striae, 3 Optical action; which are referred to in M. Sautters letter.

The colour is admitted <³> and its disadvantage (1.A.), but it is considered as a necessary consequence of that composition of the glass which gives hardness, inalterability by sea air, and diminished striae. I have no reason to doubt the quoted result of many years experience in France and I accept the subject as one of balanced considerations. Glass of less colour has come to us from France and other glass to which I cannot refer more particularly here is whiter in colour:- but the compensations hardness & unalterability of such when forming part of the apparatus are points very difficult to judge of[.]

As to Striae <⁴>. I have not made them an objection in my report I have only marked their existence & character as a record in respect of this particular apparatus[.]

As to Optical effects <⁵> I need say little or nothing[.] You will see (3.A.) that some things are to be corrected - and others (3.B.B. 3.E.) in future avoided.

I may conclude by recommending the apparatus to be accepted. I admit the considerations regarding colour hardness & inalterability - though I do not know that a white glass cannot be had possessing the requisite degree of the latter qualities - and I have full confidence that M. Sautter will cause the corrections referred to (3A) to be made⁶[.]

I am Sir | Your Very faithful Servant | M. Faraday

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