Copy of draft letter, from Sir John Herschel to John Henry Dallmeyer, dated at Collingwood Copy of draft letter, copied as part of the correspondence project led by Colonel John Herschel FRS following the death of his father. Outlining suitability of various types of glass for optical work. Gives details of theories of his reading lenses and calls attention to one of his own papers in the Philosophical Transactions of the Royal Society for 1821. 19 February 1867