Faraday Jacob Herbert   10 August 1842

Royal Institution | 10th August 1842.

Sir,

Mr. Wilkins having so far completed the light Apparatus now in his hands in which the mode of suspending and attaching the mirrors which I recommended in a former report¹ is to be adopted, as to allow of a complete arrangement of a whole division of mirrors, I have carefully examined the apparatus and believe I have reason to be satisfied with the result of the plan; for as far as I can see, in no one point does it come short of the advantages anticipated from it. I hope it will upon examination obtain also the approval of the Deputy Master² and Brethren.

Mr. Wilkins is also I believe fully satisfied with the strength of the suspension and facility of workmanship and adjustment.

The extra row of mirrors introduced between the revolving lenses and the upper part of the system of fixed mirrors answers the desired purpose of directing the light which before was lost there, exceedingly well; and that part of the frame which, in consequence, is of new construction is very good being free in it's action of revolution and yet firm in its place.

There are two changes which I would beg to submit to the consideration of the Board as of advantage in future cases. The first relates to the mirrors. The four tiers of mirrors below and the first above the refractors are all of the same size. They are numbered on the back as if intended for particular tiers, but on a close examination of all the mirrors of an apparatus, I did not find any such regularity of curve as made attention to these numbers of any use; for there was as much variety between the mirrors of one tier as between the mirrors of different tiers, I presume therefore that these numbers may be disregarded; and as the first tier above the refractors, is, in its effect, worth nearly the whole of the four tiers below, I propose that the mirrors of the five tiers be examined and those most accurate in curvature selected for this particular tier. I shall be very happy to make the examination and selection whenever required.

The second change is in the substance and construction of the ribs of the Apparatus. In the present apparatus the ribs are of wrought iron, in obedience to orders to make them exactly in the same manner as in the French Apparatus; In former apparatus constructed for Trinity House they were of cast bronze. The use of iron has caused a delay of some weeks in consequence of the much greater amount of workmanship required (being also accompanied with a corresponding increase of expence) and yet as far as I can perceive is attended with no advantage; for bronze has abundant strength and is far more readily worked into form. I therefore venture to recommend a return to the use of bronze, at least in those cases where time is an object³.

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