Much is made of the high cost of the world’s best violins, especially when they’re sold at auction. But violins are, after all, tools of a trade, and no musician would want to buy an instrument he had never played on. As it turns out, the auction houses are sensitive to that, and so, this past week, anyone who wanted a chance to play a million-dollar Stradivarius got a chance to do so (free of charge) at Christie’s in New York.