My Dear Sir,
I shall be very glad to see your observations on Heliostates. Neither Fraunhofer <1> nor Mr Potter are the first inventors of the Double mirror Heliostate. It is drawn and described in Plate 28 Fig. 400 of Vol I of Dr T. Youngs Lectures on Nat. Philosophy, published in 1807.<2>
The loss of light by two Reflexions is a great evil, and renders this kind of Heliostate of no use for observations on the extremities of the Spectrum. I have been obliged indeed to use the Sun without any reflexion at all.
In order to save light I have used [total?] Reflexions in place of Metallic ones; but what I consider a valuable Idea & one that I have put to the test of Experience is to make the ray deviate by Refraction. If we are observing parts of the Spectrum, a Single Prism will do, but if we want whole light we must correct the colour.
I do not know any subject more likely to lead to curious discoveries than the one you are occupied in viz. Microscopic Crystallisation. <3> The following method of determining the angles of the crystals may be used in your researches. Let the crystals by formed on a Plate of Glass. Hold the glass vertically & look at the sun or a candle thro’ it, and halos will under certain circumstances be seen surrounding the luminous Body. The distance of these coloured rings from the Candle gives the deviations by Refraction, from which the refracting angle, in the Inclination of certain faces of the crystal may be deduced.
The experiment is highly splendid, with small granular crystals of alum, which if I recollect rightly give these States.
I am My Dear Sir Ever Most Truly yrs
Allerly by Melrose
March 18th 1833
1. Joseph von Fraunhofer (1787–1826), optician, Munich. See R. Potter, jnr., "On a New and Simple Heliostat," The London and Edinburgh Philosophical Magazine and Journal of Science, s. 3 v. 2 no. 7, January 1833, pp. 6-8.
2. Thomas Young, A Course of Lectures on Natural Philosophy and the Mechanical Arts (London: Joseph Johnson, 1807), pl. 28, fig. 400, with associated text on pp. 425–426.
3. Possibly published as W. H. F. Talbot, ‘On the Optical Phenomena of certain Crystals’, Philosophical Transactions of the Royal Society of London v. 127, pt. 1 (1836), pp. 25–27.