The double refraction of the chitinous hair of the crayfish is positive with respect to the axis of the hair, and is largely caused by the arrangement of submicroscopic, elongated chitin particles parallel to this axis (form birefringence). Using a series of relatively unreactive liquids and fluid mixtures which permeate the chitin framework, the type of curve relating double refraction and refractive index of the imbibed fluid is found to depend greatly on the chemical nature of the fluid. Either a positive or a negative residual birefringence may be found, depending on the choice of imbibing liquid. Separation of form and crystalline elements in double refraction by means of Ambronn's imbibition technique is therefore unsafe in a system like chitin, where some type of oriented association of the imbibed molecules with the chitin framework is prevalent.