Care was taken so that convection currents were avoided. It was noted that there would be a change in the refractive index of a particular plane perpendicular to the direction of diffusion with time. The change of refractive index of a given plane at two different times can be related to the error function solution of Fick's second law. The separation of the two extremes of the change in refractive index can be linked to Fick's second law, and the diffusivity can be evaluated .