Pièce 4 - Letter from John Herschel

4 pp

The Brighton Road - Belated thanks for WW's account of Stevin's [Simon Stevin] investigations about the composition and resolution of forces. JH finds what WW says of Stevin agreeable to what Lagrange says. JH has not been employed in experiments on polarization for some months, and instead has been 'familiarizing myself with the known phenomena, and acquiring that practical habit of experimentation without which it is useless to attempt anything new'. [David] Brewster's discovery of more than one polarising axis in various crystals is a most important discovery, and completely upsets [Jean Baptiste] Biot's division of doubly refracting crystals into attractive and repulsive. JH gives a description of his inquiries and where his experimental observations differ from Brewster's: 'I observed that the phenomenon of the miniature polarised rings which Brewster spoke of in a former paper, was very different in appearance and position from what his description had led me to expect'. Instead 'of one set of ellipses, complete or nearly so seen along the axis, I saw two half sets cut off across their conjugate axes, and equally distant from the axis of the nitre prism'. Brewster places nitre among the class of salts with two axes, and JH has observed three and even contiguous sets of rings.

Second leaf torn, with loss of text.