A model of astronomical seeing with particular view to solar observations is developed which assumes the atmospheric disturbances to consist of individual turbulence elements called Schlieren. A quantitative account is given of each image motion, image blurring and scintillation as function of Schlieren properties and telescope parameters. The theory permits to explain the observational results under conditions of good seeing; furthermore, it provides a basis for the discussion of the physical phenomena in the atmosphere that cause image deterioration. As an illustration it is shown how the theory could be applied to a problem of interest to solar astronomers searching for a suitable observatory site.