TALBOT4344

S^t Leonards S^t Andrews
Oct^r 15^th 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 D^r Adamson, ² when the Spectrum which I sent you was seen. Both D^r 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 Exp^t 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 2^d 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 N^o 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 p^t I ob. 24 & Lib. II P^t 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 Exp^t 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 diff^t from mine, & to give a separate explanation of the latter, in which he completely fails.

Believe me to be Ever Most Faithf^y yrs
D Brewster

H.F. Talbot Esq^r
Lacock Abbey
Chippenham
Wilts
31 Sackville Street ⁸
London
Post Office ⁹
Brighton