Published online by Cambridge University Press: 17 January 2013
During the last half century, observations on the polarisation of the atmosphere were made by several eminent observers—by Arago, Delezenne, Babinet, and Zantedeschi; but no result of special importance was obtained till Arago made the great discovery that there existed in the atmosphere a point, or spot, in which there is no polarisation. At sunrise or sunset he found that this neutral point was 20° or 30° above the point opposite to the sun, or what we may call the Antisolar point. The name of Arago's neutral point has been given to this spot without polarisation. It is best seen after sunset. At St Andrews it is above the horizon all the day, between the middle of November and the end of January.
page 211 note * Comptes Rendus, &c., 1840. Tom. xi. p. 618.
page 213 note * See Edinburgh Transactions, 1819, vol. ix. p. 148, where the method of producing rectilineal bands, by crossing two plates of rock crystal, was first published.
page 213 note † See the section on the place of maximum polarisation, and its intensity or R., p. 245.
page 221 note * In abnormal circumstances, sometimes only 7°, 8°, 9°, or 10°, as in 1842, February 15 and 16.
page 223 note * Comptes Rendus, &c., 1840, tom. xi. p. 619.
page 226 note * See Phil. Trans., 1819, p. 146.
page 228 note * Comptes Rendus, &c., 1846, tom. xxiii, p. 234.
page 232 note * Vol. xix. part ii.
page 232 note † See Phil. Trans. 1830, pp. 69, 133, 145, 287, and Trans. Irish Acad., vol. xix. part ii.
page 235 note * Newton's Optics. 3d Edit. Book ii. part 3. Prop. vii. p. 232. See also Prop. v. p. 228, from which it would appear that by “small parcels” Newton meant solid globules of water.
page 238 note * Babinet, Comptes Rendus, &c. 1846. Tom. xxiii. p. 233.
page 239 note * See page 205.