Aqueous ammonium thiocyanate solutions as refractive index-matching   fluids with low density and viscosity
  We show that aqueous solutions of ammonium thiocyanate (NH4SCN) can be used to match the index of refraction of several transparent materials commonly used in experiments, while maintaining low viscosity and density compared to other common refractive index-matching liquids. We present empirical models for estimating the index of refraction, density, and kinematic viscosity of these solutions as a function of temperature and concentration. Finally, we summarize the chemical compatibility of ammonium thiocyanate with materials commonly used in apparatus. 
Daniel Borrero-Echeverry and Benjamin C.A. Morrison
physics.flu-dyn