Faraday to William Pigott   27 July 1861

Royal Institution 27, July 1861.

To the Deputy Master | Trinity House

I was about to write to Mr Berthon but on the receipt of your note¹ changed the direction of this letter from him to you as my best reply.

Captn. Drew Mr. James Chance & I met at Birmingham on the 23rd instance for the purpose of making certain very careful experimental observations which had been entered into & arranged by Mr. James Chance. The object was to verify the foci of the reflecting prisms of a first order apparatus as determined upon by the results obtained in relation to the Whitby & Smalls light (see my report of the 30 January 1861²) and I now give you the results. The apparatus consisted first of 13 upper reflecting prisms with an excellent lamp & flame which were adjusted to a focal point of 30mm up and 24mm aside - and also of 6 lower prisms adjusted to a focal point of 25mm up and 40mm aside. It will not be necessary to describe minutely all the operations: but each prism was examined by itself the ray from the lamp being sent through the middle of it to the distant screen where if the place of maximum light (& that best fitted for light house purposes) did not fall on the horizontal line its place was marked and then at a later period a small light was placed there and the right focus at the great lamp ascertained by the use of the focimeter. In this manner every prism was examined by itself and the results being entered upon a full sized drawing the best place for a common focus for the whole was deduced. The final results were

The previous decision as to Whitby is given in the second column - and the more recent was to the Smalls in the third column.

It will be seen that the present determination of the upper reflectors is little removed from either that of the Whitby or the Smalls. I have marked their places on a full sized diagram of the burner. The place of the French focus for these reflectors is also marked being that accepted before these investigations were made[.]

Fig. 1. Whitby

2. Smalls

3. present determ

4. French³

With regard to the lower reflectors no advantage is gained by any change for the Foci now obtained are the same as those decided upon for the Whitby & the Smalls light[.]

In these results I see strong proofs of the fitness of the alterations of the foci from the old standard which have been made during these & preceding investigations. I recommend a focus for the upper reflectors which is 30mm up and 24mm aside and for the lower reflectors one 25mm up and 40mm aside

The vertical divergences of the best part of each prism ray was noted as well as they could be;- they varied one with another from 2°30 to 3°15’. The brightest ray of light or that which went to the horizon was much nearer to the top than to the bottom of the beam. On the average of the whole one fifth of the light passed above the horizon into the sky and four fifths (being above 2°) below the horizon on to the sea:- a very good & important result. The vertical divergence of the light of the lower reflectors was not so much being only a degree. About one third of it passed above the horizon and two thirds below upon the sea.

It is intended that two diagonal wires be stretched across the optical apparatus to show the true center of the flame & also the height of the burner of the lamp. Mr Chance proposes to make the attachments for these wires such that the wires themselves shall just graze the top of the burner; it will give the lighthouse keeper a more direct reference to the place of the burner than if he has to make calculations. I approve of this change provided there be a record on the apparatus of the place of the foci in relation to the burner and also a record in degrees & millimeters of the angle which the dead level horizon & the sea horizon make with each other.

Mr Chance & I have been speaking together about the North Foreland. The focus recommended above for the upper reflectors (giving 2° of vertical divergence between the sea horizon & the shore) will with the height of that light bring the beam within a mile of the shore a very excellent result - & within that mile much weaker light will be available for the near distance. I think it would be advisable at present not to readjust the lenticular prisms of the light according to the instructions for the Smalls seeing that it is a recent good French apparatus but to limit the adjustment of that part as a whole to the sea horizon⁴.

I have the honor to be | Sir | Your Very faithful Servant | M Faraday