TALBOT2423

Allerly by Melrose
Sep^tr 27^th 1832

My Dear Sir,

As I am now about to complete my papers on the Spectrum for the Royal Society, ¹ I have been for some time engaged in identifying the Nitrous lines with those in Fraunhofer’s Plate. ² I have experienced however great difficulty in doing this in consequence of his Figure representing the Flint Glass Spectrum while my Prism is of Plate Glass. I have indeed other methods of demonstrating their general identity, but the actual comparison of each individual line in the two Spectra cannot be well made without a Flint Glass prism. My difficulties have been greatly increased by the inaccuracy of Fraunhofer’s Plate which is much greater than I could have believed; so great indeed that I am compelled to make a new Representation of the Solar lines for the purpose of investigating the phenomena of the Nitrous spectrum.–

In this perplexity, I have written this note to ask the very great favour of your lending me for a month or so, before the Sun gets dark, your largest Flint Glass Prism which I would take the greatest care of. Nothing but the absolute impossibility of obtaining such a prism in England could have induced me to make so bold a request, but as you have already put it in my power to carry on in my researches by the kind present which you made me of a Plate Glass Prism, the blame of this request must be partly thrown on your own previous liberality. I have a mass of Rock Salt on the way which I expect will furnish me with the largest good prism ever made; and I am just completing a large one of Oil of Cassia, but as this oil absorbs all the violet & part of the Blue rays it will serve only for the least refrangible half of the Spectrum.

I cannot avoid mentioning to you a very happy contrivance for increasing the distinctness of linear objects whose sides are not perfectly defined or which are rendered indistinct by some cause acting in a direction perpendicular to the length of the lines. I use for this purpose a transparent cylinder thro’ which I look at the Spectrum for analogous effects may be produced by a prism w^h a considerable refracting angle. When the length of the cylinder, the edge of the refracting angle of the prism is perpendicular to the lines, the effect is quite magical; & the method has been an invaluable resource when the lines were almost obliterated by the deposition of vapour in the tubes containing the Nitrous Acid Gas.

I have within this last week had three tremendous explosions but I have succeeded in making the gas literally black or so opaque, that a ray of the Sun’s light would not pass thro’ it. I send this to Sackville Street, ³ not knowing your address in the country. If you can grant my request, a packet would reach me safely by any Coach that goes to Newcastle, addressed to me at Allerly by Melrose to the care of M^r Hindhaugh London Courier Office Newcastle.⁴

I am My Dear Sir, Ever Most Faithfully yrs
D Brewster

H.F. Talbot Esq^e
31 Sackville Street
London
To be forwarded
Lacock Abbey ⁵
Chippenham
Wilts
Bangor Wales
Post Office
Manchester