Faraday to Peter Henry Berthon   19 December 1856

Royal Institution | 19 Decr. 1856

My dear Sir

I am now able to reply to your letter¹ respecting the lenticular apparatus, four of the twenty four lens pannels having been arranged in correct position at the Trinity house, in association with the central four-wicked lamp. Each lens pannel is 9 inches wide & 50¾ inches in height, so as to present an area of 457 square inches. The great lens (eight to the circle) is nearly 29 inches wide & 37½ in height, its area being 1088 square inches. The four lenticular pannels threw very good beams of light on to the screen placed at a distance of 43 feet; where their width was nearly 50 inches, and the dark intervals between them about 95 inches. With a uniform rate of revolution the light at this distance will continue therefore nearly half the time of the following darkness; and so on in succession. It is probable that at greater distances the light will creep a little into the darkness; but I believe that at any distance at which the light will be visible the light & the darkness will be perfectly distinct from each other. Above & below each lens are reflecting prisms[.] The eleven above give a very good amount of light, but the ray from them has an average horizontal width of 92 inches, the following dark space being about 48 inches; so that under a uniform rate of revolution the light due to this part will endure twice as long as the darkness. The four reflecting prisms below each lens pannel, give very little light by comparison; its duration will be about the same as that of the upper prisms. The effect of the whole, at this distance of 43 feet, under the supposition that one revolution is to occur in 5 minutes, would be to give for each period of 12 seconds, - 4 seconds of bright light,- succeeded by 2 seconds of feeble light,- followed by 4 seconds of darkness, succeeded by 2 seconds of feeble light:- and then again the 4 seconds of bright light & so on[.]

The glass of the lenses is excellent;- that of the reflecting prisms is green and several of them very green.- The work is excellent.

Though I have no doubt of the perfect separation of the light & darkness at any distance, still the proportion given above for the 43 feet would probably vary for quarter distances. Any variation of that kind might be ascertained by trial between Blackwall & North Woolwich station or between Purfleet & Blackwall if thought necessary.

I need scarcely observe that the light from each lens must not be expected to equal that from the great lens as above twice the number of rays from the lamp fall upon & are sent forward by the latter²[.]

I am | My dear Sir | Your faithful Servant | M. Faraday

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