The thesis of this paper† is that gelatin in photo-elasticity can be applied successfully as a potent means of quantitative stress-analysis of body-force structures capable of representation as plane elastic problems. A valid simulation of a prototype is derived, in general terms, in accordance with similarity principles and the choice of model materials shown to be vital in any particular application. Details of model construction are given and mention is made of the important influence of model thickness, temperature control, and polariscope design, on the attainable accuracy of calibration and stress patterns. The exposition is illustrated by reference to the problem of the elastic, untied, bulkhead that retains an elastic mass by virtue of its flexural rigidity. Curves of pressure distributions derived from model analyses are included.