There came a point when I’d seen sufficient historical photographs to be able to date them instantly within a decade or so from the emulsion of the print rather than by any chronological giveaway in the image from fashion or cars. Calotypes of the 1840s look as if God had dimmed the sun, for instance, whereas an 1860s glass plate exults in the light, something which by the 1880s has hardened into a kind of professionalism if one still obviously old-fashioned. Black and white photography of the 1950s has a listless, drained quality which lacks the integrity and urgency of its pre-war counterparts. The photographs of the 1980s had nothing at first, because they were new, but since the last time they were looked at they’ve taken on a greenish tint, a signal as loud if you like as that of the paperback you are shown holding new in the picture but whose pages when you reach for it now are discoloured and brittle.