Sentence:  "You make the glass invisible by putting it into a liquid of nearly the same refractive index; a <WSD>transparent</WSD> thing becomes invisible if it is put in any medium of almost the same refractive index." 
Meaning of the word marked with <WSD> tags:  See-through, clear; having the property that light passes through it almost undisturbed, such that one can see through it clearly.