Faraday to Jacob Herbert   24 October 1840

Report of Professor Faraday on the Gibraltar Lighting Apparatus, and on the Subject of adjustment of the Mirrors generally. - dated October 1840.

Sir,

It was not before yesterday afternoon that the mirrors and spherical reflectors of the Gibraltar apparatus were in their places; I have examined them and hasten to reply to your letter of the 21st respecting their arrangement.

The mirrors are adjusted, I think, nearly as well as they can be, with the present mode of attachment to the frame; but not so well as to satisfy me that nothing remains to be desired. I would pass them in this case, and will venture a few observations further on.

The spherical reflectors are of course not perfect in shape; how near they approach to possible accuracy I do not know. The light from the brightest part of the flame is pretty generally reflected back to the place from whence it issues, as may be seen by looking across through the flame to it’s image in the reflector, so that the reflectors are in that respect very well arranged. I had understood that this crossing place of the reflected rays was, in the French apparatus, made to occur about half an inch higher, as being practically better in allowing more of the reflected light to reach the refractor; but I have no experience with, or knowledge of, any French apparatus respecting this point.

Considering the Gibraltar apparatus as passed; I will now venture a few remarks upon the mirrors of similar arrangements. I have had no opportunity at any time of observing, in this very practical matter, what is the result of the experience and thought of our neighbours, the French, or even what is their system and method of constructing and attaching the mirrors; to which, as the originators of the method, their attention must have been earnestly given, I may, therefore, be considered as very presumptuous in making any remarks or suggesting any change of fitting but I hope it will be understood I am speaking in reference only to what appears in the Gibraltar apparatus.

It is of the medium ray, or the axis of the beam which is reflected and thrown forward by each mirror, and which ought certainly to be in a horizontal direction that I speak. The method of adjustment adopted by Mr. Wilkins is perfectly accurate in principle, and consists in ascertaining that the medium reflected ray is horizontal. I find, however, that it is not carried out with every mirror; but being applied to one mirror on each tier, a little gauge is constructed, by which the position of this mirror on the suspension rod is ascertained and measured, and then all the other mirrors on the same tier are adjusted by this gauge so as to have exactly the same relative position. Now though the mirrors are in this way rendered very uniform and regular in their appearance, they are not uniform in effect. The glass of some is thicker than of others; it is often thicker at one end than the other of the same mirror; the two surfaces of the same piece of glass have not the same convexity; the different glasses vary from each other in their position in the brass frames;- and other differences amongst them occur. The consequence is that the medium rays are, often, not horizontal but go up or down:- and another consequence is, that, when I adjusted the mirrors of the same tier, in a temporary manner, so as to give horizontal medium rays, the mirrors were very irregular in their appearance, one being raised a little, another lowered, and another tilted, so as to make the arrangement look very bad.

The present system of fixing and fastening the mirrors will not allow of the particular adjustment of each mirror so as to give the ray a right direction, and permit, also, of the permanent and immovable fixation of the mirror, when thus adjusted. Besides which, as now adjusted, it is found that the introduction of the hands and cloths between the tiers of mirrors to clean the surfaces, often loosens the thumb screws, and the mirrors then take any position which accident may give them. I think it would not be difficult to adopt some plan of fixing the mirrors which should allow of adjustment for each, should also make the position unchangeable and yet yield more facility to the cleaning than at present. I would suggest therefore whether it would be desirable to direct Mr. Wilkins to fit up a specimen consisting of two or three mirrors, of a method or perhaps of two or more methods, if more occur by which the mirrors could be fixed with the power of adjustment for each, and yet so as to be immovable by, and not inconvenient in, the cleaning operations of the Light House Keeper¹.

I have the honor to be | Sir | Your obedient humble Servant | (signed) M. Faraday

Royal Institution | 24th October 1840.

To | Jacob Herbert Esq. | Secretary | &c. &c. &c. | Trinity House.