Sir David Brewster in 1816 discovered that an isotropic transparent solid, such as glass, behaves as a temporary crystal under stress, and when examined in the polariscope between crossed Nicols exhibits the characteristic colours of crystalline plates.

Since Brewster's day this property has been studied by a number of observers. Clerk Maxwell showed that isinglass jelly would show the colours under stress when in its soft state, and retained them, more or less permanently, when allowed to harden under strain. To him is due the first attempt to apply this effect to the exploration of the stress system in the plane of a strained plate; experimental investigations of this nature were afterwards carried out by Carus Wilson, by Major Filon, and by Prof. E. G. Coker, F.R.S.

Neither glass, however, nor isinglass jelly forms a suitable material for the investigation by this means of stresses in engineering structures. Glass does not lend itself readily to the construction of such models as regards shape and jointing.