Faraday to James Drew   27 May 1861

Royal Institution | 27 May 1861

Examination of corrected lights by the Brethren May 1861

My dear Captn. Drew

I did not get your letter¹ until later on Saturday Night² having been away all day. I had not heard from Mr. Berthon but now reply to yours and hope you will always let me be of use if I can:- and now in answer to your questions.-

1. Dropping a plumb line from above may well help you especially in seeing whether the dioptric & catadioptric parts of the apparatus are concentric. They might not be so; and therefore I should not trust it alone for a test of the concentricity of the lamp & the dioptric part. To ascertain that, you would use the cross lines: or a gauge might be convenient being a radius with a shifting end at A and a stop at B to go into the center of the middle cotton burner. Being in place and the end A pushed against the glass a scale at S would show you at once the distance of the center from the glass on any side where you applied the gauge[.]

2. A spirit level could very easily be made to shew you whether any surface on which it was placed was level: or how much it was out of the level.

3. Should the horizon not be visible you would have to select some object known to be on the same level or plane:- in doing which a spirit level with a sight tube (horizontal) might help you. By moving right & left so that a ship at sea near the horizon may pass as it were from one frame to the contiguous frame you obtain also help in judging where the horizon is.

4. Airy I think refers only to the prisms of the dioptric part of the apparatus and these always profess to be properly curved. Gladstone refers only to the prisms of the reflecting apparatus and these are not curved in the vertical plane. The lines produced by the intersection of a vertical plane with these prisms are all straight lines[.]

5. I think such a plan as you propose of shewing the keeper his lamp and flame from downstairs might be arranged; but would not that tend in some degree to relax his watch? He could get a look at it only in one direction - not the power of examination on all sides of the flame[.]

If we met we might talk a good deal on each of these points. Writing is only a poor means of communication in such matters[.]

Ever My dear Sir | Very Truly Yours | M. Faraday

Captn. J. Drew | &c &c &c