Faraday to Peter Henry Berthon   3 July 1858

Royal Institution | 3 July 1858.

Sir

I have been to Birmingham according to your instructions¹ (accompanied as you are aware by a committee of the Elder Brethren²) and have examined carefully by day & by night the two catadioptric apparatus intended for Whitby. They offer much improvement as compared with the apparatus I last saw there, and I am satisfied with them.

As to colour the glass is excellent[.] There is very little variation in the different pieces. In some of the reflecting prisms the ray has to travel through six inches of glass, and yet these parts compared with a scale constructed from window glass, did not give a deeper colour than one inch of the lightest window glass that I could find, out of some hundred specimens. I do not expect that this colour will be surpassed by any future exertions[.]

As to bubbles; these were present in some of the pieces of glass, and irregular:- but by far the great majority of pieces were all that could be desired - Bubbles do harm according to their superficial extent but not so much as is sometimes supposed.

Striae - were present in some of the pieces and in a few were very strong indeed. In this point improvement is desirable and as in the worst cases striae occurred at one end of a piece of glass whilst the other was nearly clear it is to be hoped that such improvement will be finally obtained[.]

The mechanical working of the glass & the form was very good, being considerably in advance of the former apparatus. The work was examined closely in the day time and also at night at distances of 190, 317 and 421 feet. The rays of light issued in their proper order & right course and though the wind at night disturbed the lamps and made the light irregular still the direction of the light was always good[.]

The apparatus illumine 190° of the horizon - one of them has three large metallic reflectors in the spare space which throws the rays proceeding in their direction back towards the focus & by it (in part) on towards the refractors. Their effect in giving a superiority to that apparatus above the one without reflectors could not be judged at night because of the wind & irregularity of the lamp flame - but from the day light investigation I have no doubt that they do return useful rays of light towards the sea horizon; however I am not prepared to say in what degree[.]

The general construction of the metal frame seemed to me to be good & sound, but of that I do not profess to be a sufficient judge³.

I am Sir | Your Very faithful & Obedient Servant | M. Faraday

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