"" But if you stuck a sphere in there and made the refraction 1.33 and the refraction of the cube 1.33 the sphere should become invisible." No, you just would not see any refraction because there is no optical density boundary change." my point was... if the objects were made of the same material or equally transparent materials of the same color with identical refraction indexes the inner sphere would not be visible from outside the cube or half way into the cube / whatever in the real world. In bryce.. this won't work and especially won't work once inside the first cube as it will ONLY refract light coming off of objects outside of it (which is not really how bryce works but anyway... that's my point). Bryce's function for refraction is actually entirely reversed from real world physics... rays are sent out from the camera's location and interact with the other objects/transparancies... it's all complex and confusing and can't be compared to REAL world physics in all cases.