Visiting the National Portrait Gallery this weekend to see the (splendid) Thomas Lawrence exhibition, I noticed that the lay-out once again placed the micro-shop dedicated to selling the catalogues and other material in the middle of the exhibition. It is a separate space and clearly identified as such, but there is no way to see the entire exhibition without being channelled through it. Nothing wrong with museums and galleries striving to generate as much revenue as possible, and in Britain today perhaps more than elsewhere.

But the National Portrait Gallery went too far the other day. The terrific exhibition devoted to Camille Silvy, swansong of the distinguished photo-historian and curator Mark Haworth-Booth, had a shameless glass case right in the middle of the show, looking for all the world like one of the exhibits. Except that it wasn’t. It contained only samples of the material on sale in the bookshop elsewhere. This time, it wasn’t distinguished as being a specifically commercial space. On the contrary, it was camouflaged as part of the exhibition.

We can have some sympathy. The NPG must be desperate for every penny of revenue. But not that much sympathy. If the marketing and commercial people can’t see where they should back off, we have a serious problem of containment. Curators need plenty of courage to hold these commercial forces back, but hold them back they must. The Camille Silvy show was a grand demonstration of what public museums can do. But that one glass case was a disgrace. We need to notice these infringements or one day it will be too late.