Faraday to Peter Henry Berthon   30 January 1861

To Trinity House | report on the Smalls

Royal Institution | 30 Jany 1861

Sir

I went on Monday last¹ to the manufactory of Messrs Chance at Birmingham to examine the optical apparatus for the Smalls Light. You are aware that in consequence of certain careful & prolonged experimental inquiries, adjustments altogether new, both as to their amount and their nature, have been determined upon, and put into practice for the first time in this apparatus[.]

The manufacturer was instructed to adjust the various glass pieces by the following foci; the distances given being the foci distances above the burner and aside from its axis

all these adjustments being to the sea horizon[.]

The apparatus has been put together by Mr. James Chance with these adjustments; and being in a proper place had the focimeter set upon the burner and a true sea horizon mark placed in the distance. The whole was so true that the ray proceeding to the eye through the middle of each piece of glass passed by the focimeter at the point desired. The greatest departure was but 2mm and very few of these occurred. Further, the manner in which, as the apparatus revolved or the eye was moved about, the object at the horizon passed laterally from one panel to another, or vertically from one rib to another, showed the perfection of the adjustment of each individual piece by the harmony & consistency of the whole, though there were above 300 pieces of glass associated together[.]

At night the lamp was lighted & observed from the distance: the results accorded perfectly with the anticipations. As the head was raised or lowered each piece of glass shewed its maximum effect at the right place - its light coming in or going out as it should do in relation to the distant horizon: and I think that as far as regards the system of adjustment, the power of carrying it into effect, & finally of examining its correct application, every thing is proved to be practicable & has here been realised. The essential points now are to supply a good lamp and to provide that it be kept in good order[.]

In relation to colour & striae the glass was very good[.]

Captain Bayly² and Captain Nisbet were present at the examination[.]

It is to be remembered that the adjustments made are all in reference to the large flame of a lamp having four cottons - the utmost advantage having been taken of such portions of flame as were visible in different directions. These adjustments would not be the most perfect for a concentrated light, such as the Magneto Electric discharge³.

I am Sir | Your Very obedient humble Servant | M. Faraday.

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