One day Julia Margaret Cameron was showing John Ruskin a portfolio of her photographic portraits. The critic grew more and more impatient until he came to a study of the scientist Sir John Herschel in which the subject’s hair stood up ‘like a halo of fireworks’. At this point, Ruskin slammed the portfolio shut and Cameron thumped him violently on the back, exclaiming, ‘John Ruskin, you are not worthy of photographs!’ He was indeed smackingly wrong to dismiss her work, as visitors to an exhibition at the V&A celebrating the 200th anniversary of her birth will be able to see...