A "schliere", in German, is a local inhomogeneity. Schlieren systems, then, are optical systems designed to detect local inhomogeneities. Schlieren occur when hot water mixes with cold water, when shock waves are generated in a gas, when liquid mixtures are separated into components of different refractive index as in ultracentrifugation, and in many other phenomena involving the presence of temperature, pressure, or density gradients. However, there is no reason to restrict the term schlieren to disturbances that deflect light passing through a medium. If light is reflected off an irregular surface, such irregularities may be called 3 schlieren also. Indeed; schlieren systems, be cause they are easy to adjust, have found wide use for the testing of large mirrors.