The different apparatus used by some thirty investigators, who have induced double refraction in a liquid contained in the annular gap between two concentric cylinders, are examined with particular reference to the onset of turbulence. In the case where the outer cylinder alone rotates, curves are given which easily enable the critical gradients corresponding to the dimensions of the apparatus used to be found. The results of the examination are so tabulated that present and future workers in this field can quickly decide on the significance of the measurements obtained by any of the investigators. An explanation is put forward of the surprising fact that with either the inner or outer cylinder rotating measurements of double refraction apparently show no very sharp break between regions of laminar and turbulent flow. The value of making simultaneous measurements of streaming double refraction and viscosity is pointed out and the advantages of using an apparatus with rotating outer and stationary inner cylinder are briefly summarized. Mention is made of optical design.