Faraday to Peter Henry Berthon   11 April 1857

Royal Institution | 11 April 1857

Sir

Your letter of the 8th instant¹ requires my opinion whether a light exhibited with the 24 lens or Bishops rock apparatus would under ordinary circumstances be found as effective for the purpose of illuminating a widely extended sphere as the light from one composed of lenses of the usual magnitude i.e of eight to the circle[.] In reply I have to state that in my opinion it would not. The 24 lens apparatus aided by refracting reflectors above & below would send forth 24 beams at once in different directions towards the horizon whilst the apparatus of eight ordinary lenses would send forth only eight beams[.] I have already given the reason in my letter of the 19th Decr. 1856² why we should expect that the brilliancy of the latter should be at least twice as great as the former.

You also enquire whether the light of the former or Bishops rock apparatus will be as powerful & distinctive as one exhibited upon the ordinary system with Argand lamps & parabolic reflectors: I think not and have given what appeared to me to be the fact & reasons in my last letter of the 3rd instant³. These reflectors with their three Argand lamps will send forth much more light in one common beam than a refracting lens collecting at the utmost 1/24 part of a lamp light equal to about 14 Argands. There is nearly five times as much light in the one case as in the other & though the refractor gives a ray of less divergence than the reflectors still the improvement in that respect is not to such a degree as to compensate for the small quantity of light employed. To my eye the three reflectors much surpassed the Bishops light refractor on the evening of the 31st March last⁴.

I am Sir | Your Most obedient Servant | M. Faraday

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