Faraday to Peter Henry Berthon   15 October 1857

Royal Institution | 15 Octr. 1857

Sir

Though it would be presumptuous in me to suppose it necessary that I should give an opinion on the relative appearance of the lights exhibited last Tuesday night¹, since the Deputy Master² & many of the brethren were present to make their own observations, yet it may be expected that I should make such remarks as occur to me, not merely as regards the comparative power of the lights, but the relation of the effect to the original amount of light really concerned in the four different cases.

The apparatus at Blackwall presented on its revolution Eight lights in succession, which were observed from the top of the Lunatic Asylum at Brentwood, 15 miles in direct distance in E.N.E direction. The night was clear & moonless the stars shone well; the experiments proceeded in due order & the observations were well made. One of the lights consisted of seven parabolic reflectors with their lamps on one face. This light was very good (rather reddish to my eye) and endured for a certain amount of time, due to the amount of divergence (which is assumed as about 15°). It had a sensible size or diameter; what the astronomers would call a disc like effect in contradiction to a star light effect. Another of the eight lights was a great lens 29 inches wide & 37½ high:- it was alone, having no corresponding reflectors or refractors above or below, & was lighted by a central Fresnel lamp. Its light was very good;- white, & I think brighter, than the former one; but it did not last so long because of the lesser divergence:- it was a flash appearing & disappearing suddenly. The light was more compressed & star like than the former. The two lights were very nearly equal in effect; the first would last longest but probably be extinguished before the other by distance. Two other of the eight lights consisted each of three parabolic reflectors & their lamps arranged on one face; Buchaness fashion. These lights were much beneath the former in power; & the effect was what might be expected from the comparison of three with seven reflectors. They were arranged in a quarter of a circle, at the angle which would be taken by the introduction of eight into the circle. The two lights were of course well separated by darkness from each other. The other four lights resulted from the juxtaposition of four of the lenses of what has been called the Bishops Light associated with the same central lamp as that used with the great lens. Each of these lenses is 9 inches wide by 50½ inches high; and each is accompanied by refracting reflectors above & below. It is intended that 24 of these shall occupy the circle, & the four now in the light house were placed at the proper angle; consequently the lights followed each other quickly, but were well separated by intervals of darkness; corresponding in that respect with the observations formerly made from Woolwich³. Each light however was feeble and not equal to the Buchaness arrangement of three reflectors on the same face[.]

But besides the intensity of the light, there is the cost at which it is produced & the number of times it will recur in one revolution; the latter circumstance determining in a considerable degree, though not altogether, the time during which it will be seen. Perhaps these points may be most simply stated by assuming four light houses. If the first of these were illuminated by the system of 7 reflectors on a face, then three such faces would fill the circle the lighthouse would use the light of 21 lamps, and each flash would use the light of 7 lamps; only three flashes could be seen in one revolution. If the second had a central lamp equal, when at the best, to 13 or 14 Argands, eight large lenses with their aiding reflectors above & below could be placed around it: eight flashes would be seen in each revolution, and as the whole would only take the light equal to 13 or 14 Argands, each would apportion to itself an amount of light little more than 1⅔of an Argand lamp. If the third lighthouse were filled up with the Buchaness arrangement there would be also eight illuminations in one revolution; each would employ the light of three Argand lamps instead of the proportion of 1⅔rd of the large lens arrangement. The fourth lighthouse would contain 24 of the Bishops light lenses in the circle;- there would be 24 feeble flashes in one revolution, and each would employ light equal to [(]13 or 14[)]/ 24 i.e little more than half an Argand light. It is to be remembered however that when comparing the seven reflectors with the great lens, & the three reflectors with the Bishops lens, the reflectors give a longer light than the lenses; &, that part of the excess of light which they use, is employed in producing this prolonged effect.

The level of the top of the Asylum is higher than that of the lamps at the Trinity wharf; hence the observations were not made in the axes of the rays. As at the distance of 15 miles, a degree = 1320 feet, so, even with a difference in height of 200 feet, the observer would be only 9’ out of the axis of the ray, which where the divergence is 12° or 15° would give a very small difference, & that difference would be proportionate for all the lights compared.

The signal lights & rockets were most of them doubtful or invisible to me. Of those which were seen I could only be sure of the colour of one: which was red. One of the rockets was very good for this great distance⁴.

I remain Sir | Your Very Obedient humble Servant | M. Faraday

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