A term applied to (i) the atmospheric effect whereby distant landscape features look more hazy and bluish than near ones, and (ii) the imitation of this effect in painting. Dust and moisture particles in the atmosphere cause some scattering of light as it passes through it, the amount of scattering depending on the wavelength (hence colour) of the light. Short wavelength (blue) light is scattered most and long wavelength (red) is scattered least. This is the reason why the sky is blue and why distant dark objects appear to lie behind a veil of blue. The term ‘aerial perspective’ (prospettiva aerea) was invented by Leonardo, but the device was used much earlier by ancient Roman painters, for example at Pompeii. In the work of Italian painters of Leonardo's time, backgrounds can look artificially blue to eyes unused to Mediterranean sunlight: when Samuel Pepys visited Tangier in 1673, he observed the blueness of distant hills ‘as I have sometimes seen them painted but never believed it natural painted’. The generally hazier atmosphere of northern Europe lends itself to more subtle effects of aerial perspective. No one used it more beautifully than Turner, in some of whose late works it is virtually the subject of the painting.