St Leonards St Andrews
Octr 15th 1841
Dear Sir,
It was very rash in me to send you the drawing of the homogeneous Spectrum <1> before having verified it. I was shewing the Red bands produced by Strontium to Dr Adamson, <2> when the Spectrum which I sent you was seen. Both Dr A. and I made a drawing of it such as you have; but I find that the Prism had been moved out of its adjustment by one of us touching it, for I had made very delicate observations with it on the Sun the same day.
The Salted wick Spectrum is therefore two lines coincident with the two comprising D. Each line has a penumbra but this is owing partly to uncorrected aberrations, or imperfect adjustment or both. I can now easily distinguish the effects of the latter from those of the former. I will let you know soon whether the small line in the middle of D has a compensating one in the Salt Spectrum.–
I have looked into your Expt with the two Prisms. You shewed it to me at Lacock Abbey in 1835, <3> and I recollect hazarding an explanation of it which you have no doubt forgotten.
The Second prism ACD serves [illustration] no purpose but to assist in giving the fringes. It is better than a Plate of Glass because it has the effect of removing the 2d surface of the Plate; but I am confident you would have the very same fringes if you used a Plate whose second surface was roughened and blackened. This I think is virtually stated by yourself. Now the bands which you have observed, remarkable for their No and for their being black & white, in certain positions, are simply the bands of thin plates, and all the peculiarities which you mention are also were observed by Newton & clearly explained by him in the passages of his Optics <4> to which I referred you in my letter of yesterday <5> viz. Lib. II pt I ob. 24 & Lib. II Pt II Explanation of Fig. 7. – I cannot for a moment doubt that this is the true explanation of the phenomenon & the one which occurred to me when I saw the Expt in 1835.
You would oblige me by looking into Airy’s <6> Papers of 1840, & 1841 <7>. You will see clearly that he shelters himself from criticism by declaring that you and he saw the bands when the Retarding plate was on the Red side, which no man ever saw, or will see. Hence he is compelled to make these bands difft from mine, & to give a separate explanation of the latter, in which he completely fails.
Believe me to be Ever Most Faithfy yrs
D Brewster
H.F. Talbot Esqr
Lacock Abbey
Chippenham
Wilts
31 Sackville Street <8>
London
Post Office <9>
Brighton
Notes:
1. See Doc. No: 04342.
2. Dr John Adamson (1809–70), physician and pioneer of photography. See A. D. Morrison-Low, ‘Dr John Adamson and Robert Adamson: An Early Partnership in Scottish Photography’, The Photographic Collector, v.2, 1983, pp. 198–214.
3. Actually 1836, when Brewster stayed at Lacock before the Bristol meeting of the British Association for the Advancement of Science, which began on 22 August 1836.
4. Isaac Newton, Opticks, or A treatise of the reflexions, refractions, inflexions and colours of light ... (London: S. Smith & B. Walford, 1704). The references are to Book 2, part 1, observation 24, and Book 2, part 2, explanation of Figure 7.
6. Sir George Biddell Airy (1801–1892), Astronomer Royal. See also Doc. No: 04342.
7. G. B. Airy, ‘On a new apparent Polarity of Light’, British Association for the Advancement of Science Report, 1840 (part 2), pp. 3–5; ‘On the theoretical explanation of an apparent new Polarity in Light’, Philosophical Transactions of the Royal Society of London, 1840, pp. 225–244; Supplement to a paper “On the theoretical explanation of an apparent new Polarity in Light”, Philosophical Transactions of the Royal Society of London, 1841, pp. 1–10.
8. Readdressed in another hand.
9. Readdressed in a third hand.