The horizon, where land and sky conspire to concel the unknown, has long been a fixation for photographers. On the one hand it connotes distance, expansiveness, the epic curvature of the earth. On the other, it's a symbol of the limitation of human perspective - a locking of the infinite. At next month's PAD art and design fair, leading photography gallerist Michael Hoppen will exhibit a range of photographs of the horizon, from a rare, early twentieth century album depicting an Alaskan town with all its period oddities, to mountaineer Bradford Washburn's dramatic photographs of the Alaskan peaks. The centrepeice, though, is a rare and much sought after print by Hunter S Thompson. It shows Thompson's wife Sandy and their dog Agar on a cliff-top in Big Sur, Caliornia, in 1961. The enigmatic photograph offers an intimate glimpse of one of the twentieth century's great characters.