ONE necessity for the quantitative analysis of Schlieren diagrams, as obtained by the Thovert–Philpot–Svensson cylindrical lens method1, or the Longsworth Schlieren scanning method2, is the precise definition of the base-lines. Although it is common practice to photograph a base-line obtained through a by-pass simultaneously with the pattern, details near the base-line may easily be covered up, and furthermore an uncertainty in the position of the base-line may be introduced, since the patterns of base-lines obtained from two different places in the plane of the cell cannot be expected to appear in exactly the same position. Another accepted technique consists in superimposing the base-lines taken through the channels of the cell at the beginning of the run upon the tracings of the final patterns, so that those parts which do not show an elevation coincide. It is, however, a frequent occurrence, in electrophoresis experiments, for example, that a pattern spreads out to such an extent that no part of it can be considered to indicate the position of the true base-line. The device described here permits analysis in cases such as this, while in all others it provides a useful check.