Philosophical sounding question: How can you see somthing that is clear like water or glass? Reflections, yes, but more so, distortions of light coming through. The distortion is caused by light being bent when it changes index of refraction. This site will teach you how to mix up a solution with the same index of refraction as Pyrex. When submerged, the glass will be invisible.
http://gr5.org/index_of_refraction/babywess.jpg
http://www.exploratorium.edu/snacks/disappearing_glass_rods/index.html