James Timmins Chance to Faraday   12 November 1861

Bournemouth 12 Nov: 1861

My dear Sir,

I enclose a report in regard to St. Anne’s Lighthouse, Milford Haven¹. The alterations have converted a very inferior apparatus into one of the best ones.

You will perceive, in my remarks, an allusion to the defective curvature of the reflecting surfaces in this light, as in the North Foreland one. They are far too converging, so that in adjusting the tower prisms to our present standard arrangement - (viz. 25mm above the burner, & 40 mm in front of the axis) - the highest part of the prism sends all its light to the sky: - thus supposing F to be focus of the prism ABC, then if m be the middle of the face AC, the present plan is to make mfo, which is the axis of the pencil of light (passing through the middle of the prism), to pass though the point O - 25mm up & 40mm - before the axis.

But in consequence of this excessive defective convergence, the light from the horizon through the highest part towards a falls below the burner - i.e. all light from the flame transmitted by the part a goes to the sky.

Also the part c corresponds to the weak part, or point of the flame: but then all its light falls somewhere on the sea.

If the prism be too diverging matters are reversed; viz: the part a corresponds to the top or weak part of the flame; and it is the lowest part c which then sends all the light of the flame to the sky.

The question is, whether it would not be better to adjust these extremely converging, or diverging, prisms, so that every part of them shall be doing some work upon the sea.

This may be done very simply by our fixing upon some minimum height above the burner, below which the ray af (in converging prism) - or the ray cf (in a diverging one[)] - shall never be allowed to fall.

The focimeter would then be of this form - with an additional up minimum measure (x), below which the image, seen in the top or bottom of the prism, shall never fall - still using the 25 mm for the regular ordinary adjustment.

Yours very truly | J.T. Chance