TALBOT1349

31 Sackville S^{t 1}

Dear Sir

There is one circumstance presented by the glass films which we examined yesterday, ² which still needs explanation nor do I see how it is deducible from the Newtonian rings ³– It is this: if the daylight reflection from one if viewed thro’ another held at any distance (tho’ the nearer the better) is separated into coloured bars. The same is more distinctly seen if the light be transmitted thro’ the first film and reflected from the second. Neither of them separately shew any colour whatever in the daylight. I have thoughts of drawing up for Brewster’s journal a short account of this and several other phenomena presented by the Monochromatic Lamp; as I am not acquainted with D^r Brewster perhaps you will have the kindness to transmit it. ⁴

Will you favor me with Cap^t Kater’s ⁵ direction –

You were of opinion that the variations in the thickness of the film were appreciable by the spherometer – and could be found equal to the length of one undulation of yellow light between each of the obscure bands – This yellow light is identical with the rays named D by Fraunhofer and he has given the length of their undulations with the utmost accuracy in his Kurzer Bericht 21 ¾ millionths of a Paris Inch.– p. 359. ⁶

The spectrum afforded by the Red Fire of the Theatre ⁷ is one of the most beautiful of phenomena, from the number of maxima of light which it presents.

In the Red – First, a streak beyond the limits of the ordinary spectrum due to the nitre, followed by a considerable space of darkness: then very numerous crowded bright streaks with dark space between.

One orange streak

One yellow (due to the sulphur)

Three green, one of them very broad

One very light blue, & several fainter

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