Faraday to Jacob Herbert   13 May 1842

Royal Institution | 13th May 1842.

Sir,

In accordance with former instructions I have been carrying out as quickly as Mr. Wilkins convenience would allow the experimental trials of a method of fixing the mirrors of a Catadioptric light in manner different to that adopted in the French apparatus or in the former English arrangements. I have brought the result to a state satisfactory to myself and there is now at Mr. Wilkin's manufactory an arrangement of four rows of mirrors arranged alternately upon the French plan and that which I propose to substitute for it for the observation and judgment of the Deputy Master¹ and the Board. To render it quite intelligible I beg to add the following explanations.

The mirror frame generally consists of a ring six feet in diameter to which are attached eight equidistant uprights, these are crossed by bars or arms which are again connected by round rods proceeding from one arm to the neighbouring arms on the same horizontal line. In this manner the rods form circular horizontal rings to which the mirrors are attached, and each tier of mirrors requires two of these rings to hold them in their places[.] The change which I propose relates to the position of these rings and the mode in which the mirrors are attached to them.

In the sketch 1, 2 and 3 represent three mirrors, 1 is attached by the French method; 2 by that which I propose - a and b are the places (seen in section) of the sustaining rods of the French Apparatus; a socket with a projecting arm is fixed on a; c is a clamp attached to the arm of the socket by an adjusting screw and it holds the mirror by a nip between the tongue of the clamp and a screw working in the upper part - d and e are two similar clamps. The clamp c lays hold of the mirror by the middle of the inner end, the clamps d and e lay hold of it on either side near the outer end, thus giving three places of support for each mirror.

In the method I propose m and n are the places of the sustaining rods;- they carry as before sockets with arms arrang'd as in the sketch; and the connexion with the mirror is made at once by a jointed screw which being soldered on to the back of the mirror passes thro' an enlarged hole in the tongue of the socket and is held fast by two adjusting nuts. The relative form, size and place of the French attachment and mine, is seen in the enlarged drawings.

The great object and effect of the change is first of all to avoid that loss of light which now occurs with the French plan, and did occur with the former English arrangement (as it is at the St. Catherine's Light House &c) by the position of the sustaining rods. It will easily be seen that the rod a stops much incident light being an opaque obstacle in its course. The clamp c also where it occurs, stops more light which consequently never reaches the mirror above;- and then again other light which has reached the mirror and been reflected, is stopped by the rod b a second loss being thus occasion'd. In the French arrangement all those rods and clamps at the ends of the mirrors towards the light, obstruct in the above manner, but those at the ends outwards from the light obstruct only at the lower and principal mirrors and not at the uppermost ones,

In my arrangement the rods, clamps, &c are placed entirely within the shade of the mirrors as seen in the shade of mirror 2 so that they cannot stop any light which can pass the mirrors themselves. This is done by bringing the rods closer together and nearer to the back of the mirror, by altering the form of the socket and its tongue, by making the tongue turn in the opposite direction, by dismissing the clamp and it's screw and substituting a jointed screw for it, and by the choice of the place to which the screw is attached.

A second consequence of the change is that much readier access of the hand to the space between the mirrors is obtained in the act of cleaning them. The way in between the mirrors from the light side is, in the French plan, much blocked up by the suspending rods and clamps.

I find at Mr. Wilkins manufactory that the nip of the screw in the French clamp upon the edge of the mirrors has in two or three cases broken the glass. This effect is entirely avoided by the proposed mode of suspension.

The adjustment of the mirrors into position is found to be very easy and convenient on the plan I propose. The holes in the tongues are made so much larger than the small screws which pass through them, as to remove all unfit restraint, and allow for all irregularity likely to occur in workmanship; and yet without in the least interfering with the final adjustment and fixing of the mirrors.

The mirrors are exceedingly well and firmly held in their places.

The positions of the mirrors in the drawings are those of the French arrangement of the first, second, and third tier from the refractor upwards; the scale being one half. The differences of interval &c depend upon circumstances connected with the form, size and place of the light, but the obstructing effect of the suspending rod is the same in kind wherever it occurs.

In proposing this change I am not aware (though I have sought for it) that I am likely to introduce any one circumstance in which the French plan has the superiority; though I should have been prepared to have submitted to two or three if needful so that I might secure the paramount object of sending all the light possible, out to sea; but as to a test much more acute and practical than my own judgment, I now beg the favour of the submission of the plan to the judgment of the Deputy Master and authorities of the Trinity House².

I am | Sir, | Your very obliged and obedient Servant, | (signed) M. Faraday

Jacob Herbert Esq | &c &c &c | Secretary.