Copy letter, from Sir John Herschel to John Tyndall, dated at Collingwood Copy of letter, copied as part of the correspondence project led by Colonel John Herschel FRS following the death of his father. Likes John Tyndall's explanation of sky's blue color. Has difficulty with John Tyndall's explanation of polarization of skylight. Comments on absorptive powers of vapor and liquid. 5 January 1863