Faraday to Peter Henry Berthon   10 October 1859

Royal Institution | 10 Octr. 1859.

Sir

Referring to the copy of a letter to Messrs Chance which I sent to you on the 8th instant¹ and which had reference to your communication of the 4th² to me have received replies from M.M. Chance³ and I am now in a condition to answer your enquiries. The first modification proposed consists in an increase of the number of the catadioptric prisms each prism being made smaller than before. The increase of number is considered favourable because the planes of the more numerous prisms approach nearer to the form of the curve of a perfect optical apparatus than the planes of a lesser number of larger prisms do. Considering the size of the flame & the divergence required by the light house service I am not aware that this is of much importance but no harm can be produced by it. Another effect of increased number is that the rays would have to travel though a lesser thickness of glass and that, therefore, less light would be lost by absorption. This is undoubtedly true and in that respect the change would be of advantage. No other point regarding the number arises, either as to strength, accuracy, or fitting: the change would be rather disadvantageous to the maker as there would be more faces to work. Messrs Chance tell me that of late years the first order lights have been nearly all constructed upon the new section containing 18 & 8 prisms instead of 13 and 6.

The second modification is to construct the apparatus with 12 faces to revolve in six minutes, instead of with 6 faces to revolve in three minutes;- the intention being to have half minute intervals. I do not find that there is any ground of objection to a revolution of the whole apparatus in either three or six minutes, independent of its effect on the light: and if the light is to be limited to half minute intervals, with either 12 or 6 faces, then the effect will be as follows. The flash from 12 faces will have only half (or nearly half) the intensity of that from 6 faces, but it will last twice as long; therefore the choice is between a quick strong flash with a long interval of darkness, & a slower weaker flash with a shorter interval of darkness. As to the nautical value of these two results I am not a practical or proper judge; though I can understand that the increased brightness in the one case is an advantage; and that the increased duration in the other may also be a benefit at times. Of course half minute time may be given by apparatus with either 6. 8. 10 or 12 faces. I conclude there are revolving lights in existence by which the question of time may in some degree be judged of - i.e where a face passes though 30° or 60° of revolution in one interval but I must refer that point to the decision of the Elder Brethren.

I return the plan & letters which you sent me⁴[.]

I have the honour to be | Sir | Your Very Obedient faithful Servant | M. Faraday

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