A wide range Schlieren system

The Schlieren optical system is well known to be a simple and sensitive method of displaying variations in air density, such as are found, for instance, in the flow of air through a wind tunnel. Normally, a choice must be made between high sensitivity to show fine detail and long range so that main features are not lost or distorted. Methods are given here whereby both high sensitivity and fine detail are obtained simultaneously. It is even possible to obtain unambiguous measurements through glass windows not initially designed for Schlieren work; or to show fine striations in all parts of a sheet of glass which has also larger surface irregularities.