Am trying to match the refractive index of solids such as glasses, cryolite, or silicones to glyerol (ri=1.473), water (1.33), ethanol (1.36), propanol (1.47), or mixtures of the same in other to make them "invisible" when immersed in the liquid. This is for a photomicrography application, so the standards are fairly exacting.

I find that even when the RI's are a fairly close match, e.g. borosilicate glass (1.474) and glycerol, crushed glass (~0.2mm) is still faintly visible. However, larger pieces (~2-5mm) are nearly invisible.

Is this due to angle of incidence and the many facets on crushed glass particles? I.e. at low angles light reflects even if the RI's match? If so, is a spherical shape the best way to minimize reflectance?

Pure cryolite sand is said to do the same in water--i.e. a large crystal disappears, but synthetic cryolite sand (white when dry) is still quite visible in water.

Another combination is magnesium fluoride in isopropanol (ri's ~1.477), but if I ground up the MgF2 into sand, would the same thing happen?

How about teflon AF particles (~1.33)? Or photographic grade gelatin? Or optically clear silicone?

Fused quartz spheres (1.454) might be easier to match, but seem impossible to obtain.

Would like size to be ca. 0.2mm.

Can anyone suggest other combinations?