LONDON.- An extraordinary letter from William Blake to the painter Ozias Humphry describing in great and powerful detail his famous water-colour, The Last Judgement, is for sale as part of the Roy Davids Collection of Papers and Portraits at Bonhams on Tuesday 29 March 2011. Among the longest and most important of Blake’s known letters it is effectively a literary essay on his painting and is estimated at between £50,000 and £60,000. Letters in Blake’s own hand are very scarce – only 90 are believed to exist. The originals of many are hard to trace and, of the known letters, most belong to institutions. In the past 35 years only two documents by Blake – a one page letter about his health and a receipt – have appeared at auction. The Last Judgement was commissioned through Humphry for the Countess of Egremont to hang at Petworth House where it remains to this day. Blake’s letter explains the