James Timmins Chance to Faraday   19 November 1860

Hamstead | Birmingham | Nov. 19. 1860

My dear Sir,

Last Saturday I sent you a memorandum of the alterations at the South Whitby Lighthouse¹.

In referring to the reflection I used the term “focal line”. I am sure that you will understand my nomenclature. In regard to any particular focus, if the prism is formed correctly, that focus (whether too near or too far) lies on the ray passing through the middle of the vertical section of the reflecting prism. Hence for brevity’s sake, I term this ray the ‘focal line’.

The only possibility of error at Whitby would be such as might have arisen from the upper stays being unfastened during the adjustment, whereby the apparatus might have inclined slightly, (though for all practical purposes, immaterially), in one particular direction

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Lens Experiments

I am quite sure that you will like to see the effect of each lenticular band.

The adjustment of all below & above the middle belt depends upon the choice of the brightest Colour; which colour is clearly defined on the screen.

I have found singular results so far - if I am guilty of no error.

Height of focus above the burner in the axis

So that the orthodox height is right for No. 9 - but for all the others the brightest light is thrown away.

Yrs very truly | J.T. Chance