The vial becomes clear when it is immersed in glycerol, since glass and glycerol have similar refractive indices.
Tissue clearing using Visikol HISTO is similar in this fashion. Biological tissue has proteins and membranes which have a high refractive index, and water which has a lower refractive index. That makes each cell and cellular component refract light as it passes through the specimen. If you could remove the water and replace it with something that matched the refractive index of the proteins/membranes, the biological tissue would become transparent. Visikol HISTO replaces the water in the cells, and matches the refractive index of the tissue components. The chemistry of Visikol HISTO is specially tuned to penetrate into biological tissues, closely adsorb to proteins and membranes, and replace the water, leading to homogenization of the refractive index of the overall tissue, and thus inducing transparency.
Of course there are other clearing techniques, but they all work to essentially homogenize the refractive index in one way or another. CLARITY does this by removing lipid components, allowing for a refractive index matching solution to penetrate the tissue and clear. CUBIC/Scale work by denaturing the proteins in tissue so the inner parts of the proteins and membranes become soluble to the aqueous clearing agent (which is a higher refractive index than water). However, CLARITY and CUBIC/Scale cause irreversible changes to the tissue, and are thus not ideal, since the underlying structure of the proteins and membranes is disrupted. Visikol HISTO does not cause this, and hence is fully reversible. After clearing a tissue with Visikol HISTO, one can wash the solution back out of the tissue, and rehydrate it, restoring the tissue to its original state. This is very desirable for researchers using important and valuable specimens, since it allows them to do follow up work with the original tissue specimen and validate their 3D imaging results using conventional techniques.