A large-area interferometer is described which is based on schlieren system and designed to be used subtractively by a double exposure method. The beam is split using front and rear surface reflections from a small-angle prism. On the double exposure interferogram both the fringe spacing and shear are variable. The aim is to subtract not only aberrations and windows, but principally end-effects, which otherwise introduce disproportionately large errors in the study of hot test objects. To this end burners and heated pipes of different lengths, but identical in every other respect, are used. Various methods of subtraction are discussed and illustrated by examples taken from a study of the effect of electric fields on heat transfer from flame gases.