Unpublished letter, 'On some new optical phaenomena' from John Herapath to Davies Gilbert Observations made in early 1819 in consequence of a paper that Herapath read by [John] Knox in Philosophical Transactions for 1815 titled 'On some phenomena of colours, exhibited by thin plates'. Herapath sought to determine whether the effect observed by Knox was caused by an interference of light or was due to Isaac Newton's theory of fits of easy reflection and transmission. He found the latter to be the case. Includes four figures on separate pages. Subject: Optics Written by Herapath in Cranford [London]. Received by the Royal Society on 17 June [1830]. 15 June 1830