Faraday to Peter Henry Berthon   8 July 1858

Royal Institution | 8 July 1858.

Dear Sir

The Catadioptric light apparatus intended for the Bishops rock lighthouse having been erected at the Trinity house, so as to be accessible in all directions & capable of examination, I proceeded yesterday to inspect it both by day & night. The points which admit of such examination are Colour,- bubbles,- striae,- workmanship, & optical action of the parts. When these are all right, there is no doubt that the effect at a distance will be good[.]

As to colour the glass was greener than I expected to find it. It was sensible in the refractive & chief part of the apparatus, and much more in the reflecting prisms above and below. There the colour in many pieces equalled Nos. 6 & 7 or the deeper shades of the experimental scale I have before referred to. Improvement in this respect is desirable and attainable.

In respect of bubbles the glass was very clear indeed. There were a few here & there, but the apparatus is in that point of view unexceptionable.

There were no large or strong striae, but numerous small ones, often occupying the whole of the glass and lying horizontally in it from end to end. Their smallness & the manner in which they are disposed, is evidently due to the way in which the glass has been gathered & drawn out. As far as they can act they tend to disperse light in a vertical plane; but it is difficult to say to what extent this may take place, & whether the effect at a distance is importantly affected by them or not.

The optical action & distribution of the light is not so good as I expected; considering the results with some former French apparatus. As the eye travels up and down near the apparatus, between the top & bottom, the flame or other object in the focus appears in the ribs of glass in the proper order:- but at 15 or 20 feet off, many of them present the luminous object in the reverse order. Closer examination shows that this is because the secondary foci of many of the ribs are at distances not greater than from 8 to 20 feet instead of being on the horizon;- the rays from these pieces do not therefore proceed in a parallel direction to the horizon but diverge after passing these foci. The effect on the screen at different distances is manifested by the production of light & dark bands. A dark band originates 5½ inches below the level of the focus (at the line where the 11 inch central rib joins the one beneath it); which is common to many of the pannels & does not disappear at any distance which I can obtain in the room. I cannot tell how far these circumstances would affect the result at a distance and it is right that I should call to remembrance experiments made many years ago (& reported upon) in which two pannels at Purfleet, were examined & compared from Blackwall wharf¹, the one being a well wrought French pannel & the other a pannel from Newcastle having the condition described above. The latter did not present the inferior condition which was expected & I concluded that in that case the imperfect workmanship of each piece or rib was compensated for by the overlapping of the rays of all the 17 pieces of which the pannel was composed.

The workmanship of the frame of the apparatus appears to be very good².

I am | My dear Sir | Most faithfully Yours | M. Faraday

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