Although the photosphere is a uniform region for scales greater than the granulation, the fact that the magnetic field strength falls off less sharply than the gas pressure leads to strong magnetic influence at greater heights in the solar atmosphere. This magnetic influence leads to non-uniformity and fine structure in the chromosphere and corona. The existence of such structure has been deduced mostly from measurements of photospheric phenomena; in particular, from measurements of photospheric velocity fields (Leighton, Noyes & Simon 1962) and of photospheric magnetic fields (Bumba & Howard 1965). The determining factor would thus appear to be in the photosphere; but visible effects only are produced in the chromosphere and corona. In recent years, high resolution filter photography has enabled us to recognize different regions of the chromosphere, where qualitatively different structure is associated with distinct magnetic field patterns. This progress has been possible because of better Lyot filters, better films and better observing sites; the spectroheliograph has always been limited for high resolution work by the finite slit width and the difficulty of accurate guiding during the long exposures.
