That happened, for me at least, with Voices from a French Village, last night's feature on Radio 4, in which Gillian Tindall used a chanced-upon bundle of love-letters addressed to an inn- keeper's daughter as the starting-point for an evocation of life in rural France in the half-century before the First World War. In one sense, the programme was completely successful: Tindall showed you the ambitions and limitations that shaped the woman's life, along the way creating a strong sense of involvement in her story. But it could never evoke any concrete-seeming picture of what was going on, because I simply don't have a strong enough idea of what a small French town of the period might have looked like. Radio relies on images recalled: however much the imagination transforms them, they have to be there in the first place.
Which is not to say that radio is inferior to TV. But its strength - and this is something you sometimes feel the programme makers forget - lies in what you hear: in the precise meanings we attach to sounds and atmospheres, and in language.
If you want an illustration of this, try the current morning reading, standing in for Yesterday in Parliament on Radio 4: Captain Joshua Slocum's account of a pioneering voyage undertaken between 1895 and 1898, Sailing Alone Around the World. He doesn't conjure up any clear picture of Tierra del Fuego, or fill your nostrils with the salt tang of the sea. What catches the ear is the rhythm, the effortless formality and the self-assurance of Slocum's prose (which William Roberts' mellow American burr suits down to the ground) - nicely summed up when he describes his emotions on setting out from Boston: "I felt that there could be no turning back, and that I was engaging in an adventure the meaning of which I thoroughly understood. I had taken little advice from anyone, for I had a right to my own opinions in matters pertaining to the sea." The reading is thoroughly addictive; but it's the words, not the pictures, that do it.