Jacob van Ruisdael’s ‘The Hekelveld’, in Amsterdam in Winter (around 1665-70), is in Agnews’ London Art Week exhibition, Landscapes and Cityscapes, from the 17th to 20th century Courtesy of Agnews The contemporary art market yo-yos with the stock markets and whims of speculators, the auction results of its young stars shoehorned into indexes that dress them up as investment material. But Old Masters are a more capricious beast. This is not a constructible market with a continuous supply from malleable young artists; instead, there are fallow years, offering only dull landscapes filled with depressed cattle, and ropey boss-eyed portraits, when dealers will tell you (from their second homes in Provence) that the end is nigh and supply has dried up once and for all.