Faraday to Peter Henry Berthon   18 May 1857

Report<qr>Royal Institution | 18 May 1857

Sir

I have been to Birmingham¹ and examined the apparatus for the Lundy light by day & night, and hasten to make the following report. The apparatus which is to revolve consisted of eight principal lenses occupying the middle part;- eight upper holo catadyoptrical pannels each acting virtually as a lens:- and a series of lower prisms which refracting only in a vertical plane gave a continuous light (though feeble) all round the horizon[.]

The colour of the glass is good & regular throughout being very small in quantity even in those parts of the upper prism where the light has by reflection & refraction to pass through six inches in thickness of the glass - it is of a pale bluish green in character[.]

The glass is remarkably clear from bubbles.

In respect of the perfection of form & fitting of the different pieces of glass I have to observe that the eight chief lenses are exceedingly well worked. My close inspection discovered a deficiency of true form in the central piece of two of them; these are to be changed. Excellent machinery for the grinding of the glass is in course of erection and it is in the first pieces ground by this machinery that I find the condition, it did not occur afterwards: and now that the parties know how to observe this machinery it is not likely to occur again. - The upper holophotal pannels or lenses are also well shaped & worked. Some of the lower zones prisms i.e of those at the bottom of the apparatus are not so perfect. The fact is that because the place of the burner shuts off in all cases the true focal point from these lower zones, a higher focal point has to be taken for them. It is well known how little power these zones add in any case to the whole amount of light. If however such a source of light as that supp[o]rted by the Magneto electric machine ever came into use then the true focal point will be as important to the lower as to the upper part of the apparatus; & then the lower pieces must be worked in accordance with the upper.

Striae. On the whole the glass was good in respect of striae;- as good as any apparatus that I have seen.- Since parts were much marked by fine striae I endeavoured to ascertain whether, at night time, they produced any sensible injurious effect but could not distinguish any such[.]

Several of the chief lenses were examined by the sun’s rays and gave very good foci. The two that I spoke of as having the centres slightly deformed did not shew any ill effect here; neither indeed could they, the imperfection being only over a small surface[.]

The middle & the upper part of this apparatus revolves;- they are fixed to each other so as to revolve together;- the upper tier of lenses (or parts of lenses) may be made to coincide with the middle or chief tier so that the lights shall unite or they may be moved more or less on one side so as to give separate flashes. In the present case they are on one side about 7° and give 16 lights in the circle i.e a principal flash then darkness - then a second flash & then a longer darkness[.]

A four-wicked central lamp was placed in the apparatus and the effect examined at night (which being moonless was favourable for the purpose). Different distances were chosen:- the following results were obtained at 100 yards[.] The light of the eight chief lenses was excellent, very regular & bright in every part, & apparently uniform in the whole course of its path across the eye:- its width was 26½ feet at this distance showing a divergence of 5 degrees or a little more. The light from the eight upper lenses was very excellent in character it was not so abundant of course as that from the chief lenses, nor so steady because the rays forming this light come from the top or forked part of the lamp flame & take up a slightly varying character accordingly. The width of the flash or beam was again 26½ feet or 5 degrees as it ought to be:- The dark interval between these two flashes was 12 feet or 2⅓degrees nearly: the larger dark interval was 171 feet or 32½ degrees.

I measured the proportion of light just formed by the upper & the lower lenses:- it was as 8 to 14 nearly that is, the upper tier of lenses (for such they vertically are) gave rather more than half as much light as the chief or middle tier[.]

The upper lenses (which I believe are the proposition of Mr Stevenson) appear to answer their purpose perfectly. Whether the present arrangement of them & the lower is the best is for the Trinity Board or the Mariner to judge[.] At present there are sixteen lights in the circle: the lights vary being alternately larger & less, & the intervals vary being alternately larger & shorter; the intervals can be varied in any degree. The lights also may be varied to a certain degree, for if they overlap more or less, the effect will be a beam of a certain brightness, then an increase to greater brightness,- then a diminution, & finally a disappearance: or if the lenses coincide there will be eight very bright & equal lights, with eight long equal intervals. When the arrangement is decided upon, it can be made & rendered permanent with the utmost facility²[.]

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

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