Allerly by Melrose
Septr 27th 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 wh 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 Mr Hindhaugh London Courier Office Newcastle.<4>
I am My Dear Sir, Ever Most Faithfully yrs
H.F. Talbot Esqe
31 Sackville Street
To be forwarded
Lacock Abbey <5>
1. D. Brewster, ‘Observations on the lines of the Solar Spectrum, and on those produced by the Earth’s Atmosphere, and by the Action of Nitrous Acid Gas’, Transactions of the Royal Society of Edinburgh, v. 12, 1834, pp. 519–530.
3. 31 Sackville Street, London residence of the Feildings, often used as a London base by WHFT.
4. Joseph Hindhaugh, was listed as a coach proprietor in the 1833 Newcastle Directory.
5. Readdressed in another hand.