Sackville S^{t <1>}
Jan^ry 12^th 1826

Dear Sir

Enclosed is a sheet from the Scientific Gazette of August 27^{th <2>} containing an account of the effects of Muriate of Lime on vegetation, which you wished to find again. Is Muriate of Lime the same with Chlorate of Calcium as there stated? I thought it had been the Hydrochlorate. If the statement is true, the main point is to find the proper proportion of salt, for I have no doubt that too much of it would be fatal to the plant.

I have repeated my experiment ^<3> this morning, and with the same result as before, or even more decidedly, as the coloured bands were in some circumstances strongly developed – What these favorable circumstances were, I was unable to define, nor can I at all explain the phenomenon at present. The bands frequently appear on one portion only of the film. I intend to place the films between two frames [illustration] of card, to keep them flat; as their concave shape complicates the phenomenon very much.– You have perfectly understood my meaning, & I can only attribute your not seeing these bands to the use of candlelight. ^<4> I shall endeavour to trace the origin of their anomalous appearance, and trust that you will [be?] daylight remark the same. If you lay one film on another, by [illegible deletion] its own weight, you will hardly fail to make colours; then removing them to a distance without taking the eye off them, you will perceive that they continue to be visible.

I am much pleased with your ingenious suggestion for comparing the length of oscillation of the Red and Yellow Rays, and I think it cannot fail of being successful.

I doubt whether the bands seen by transmitted homogenous light ought ever to be perfectly black; but they ought to be so with reflected light: & I think you will find that the bands seen by reflection one a glass film are much darker than those seen by transmission.

I am aware that the yellow flame of the Spirit Lamp contains much green and blue; but these may be neglected as they disappear in most experiments from their want of illuminating power: or they may be removed by a pale yellow glass – but it appears to me that the yellow light is perfectly homogenous.

I am very glad that you agree with me in the fact of there existing a red light of inferior refrangibility to any in the Solar Spectrum; in the absence of a regular measurement this coincidence of opinion gives considerable probability to the fact.

I made the following exp^t – The light of a candle was transmitted thro a narrow crevice [illustration] AB, and the daylight thro’ its continuation BC. The day was rather cloudy. The spectra afforded by the two lines corresponded in their colours, with this exception, that the Red was considerably more prolonged in the Spectrum of the Candle. Probably however this would not be the case if Solar Light were used – I have observed this Exterior Red Ray, in some other instances, viz. in the Flame of Alcohol burning intensely in an Argand Lamp; ^<5> I consider this circumstance very singular and deserving investigation. At the same time the faint streaks (or maxima) which are usually situated in the Blue and Green, disappear: so that the spectrum is identical (except in brightness) with that given by sulphur deflagrated with Nitre:

I remain Dear Sir Yours most truly
H. F. Talbot

I have omitted to state a third method of obtaining this Exterior Red Ray; it is simply to soak a cotton wick in Nitre, dry it well, and use it in the spirit lamp.– What do you think of the passage I quoted from Fraunhofer’s work? ^<6> I enclose two films that exhibit the phenomenon I mentioned; but they want cleaning first in some acid. The bars which appear with 2 films are sometimes zigzag thus [illustration] at least with some films. I attribute it to irregular thickness tho’ not without some doubts. N.B. The colours visible were Red & Green & Brown.

J. F.W. Herschel, Esq.
56 Devonshire S^t

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Notes:

1. 31 Sackville Street, London residence of the Feildings, often used as a London base by WHFT.

2. ‘Muriate of Lime, a valuable Manure”, The Scientific Gazette or Library of Mechanical Philosophy, Chemistry and Discovery, no. 9, pp. 125–126, 27 August 1825. The journal was edited by Charles Frederick Partington, and was issued from July 1825– 4 February 1826.

3. On 3 January, WHFT had made some observations of Newton rings with a very thin glass. [See Doc. No: 03149].

4. See Doc. No: 01354.

5. Introduced by Ami Argand (1750–1803) in 1784, this improved oil lamp provided a more steady light source with less accompanying smoke.

6. See Doc. No: 01348.