Faraday report to Trinity House   12 March 1860

Royal Institution, 12th March 1860.

I have been engaged in some observations upon the divergence of the electric beam, when emerging from a perfect lens, of the first order light apparatus, the electric spark being in the true focus. The lens was 39 inches high and 28 inches broad. I have no doubt the divergence in a vertical direction will be sufficient; indeed, it was proved to be so at the South Foreland. That in a horizontal direction will be less; both because the width of the lens is less than its height, and the width of the electric light is also smaller than its height, the two carbons used being one above and the other below. I fear, therefore, that the flash in the distance may, with the present apparatus, be of very short duration, though intense in power.

In a letter, dated 3d April 1857¹, is noticed the appearance, at Woolwich, of the Electric Light placed in the focus of the “Bishop’s” apparatus at Blackwall; and there the beauty and briefness of the light is referred to. If the illumination seaward is only over a limited number of degrees, then the light on the land side may, by small reflecting surfaces, be thrown seaward, so as to prolong in some degree the duration of the flash².

(signed) M. Faraday