Faraday to Peter Henry Berthon   13 June 1860

Royal Institution | 13 June 1860

Sir

On the 11th of June I examined the light apparatus to which your letter of the 2nd refers¹[.] On all points - as regards Colour of the glass - Striae - bubbles - Workmanship of the pieces and association of the parts into two great wholes they were with one small exception equal and in respect of adjustment superior to former apparatus, so as to obtain my entire approval. The reserved point was this, a part of the glass at one of the centres of the eight holophotal lenses of the larger apparatus was a little deformed so that the focus of that part was not quite the same as that of all the rest of the glass. The space affected was not larger than a sixpence & the only result was to disturb a little the direction of the ray at the central part of the beam not however throwing it out of effect but simply diffusing it. The effect is so little (being not really injurious) that I did not think I ought to object to it, though it is not to be repeated[.]

The lamp in the fourth order apparatus had one larger wick & a central deflector but a central wick in place of the deflector would have been a great advantage as regards those rays which descend from the flame to the lower part of the apparatus the deflector offers much obstruction for none of them can pass from the further side through it across the flame[.] In my opinion the suppression of the central flame & the substitution of an opaque mass of metal in the shape of a deflector can never be an advantage².

I have the honor to be | Sir | Your faithful humble Servant | M. Faraday

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