Disappearing Glass Rods illustrates the effect created by the different properties of two kinds of glass. Two bundles of glass rods -- one made of flint glass and the other of Pyrex -- can be lowered into a tank of immersion oil. Because the flint glass is the same optical density as the oil, the rods bend light that passes through them in the same way as the oil,and the flint glass seems to disappear. The Pyrex glass rods have a different optical density and so can still be seen in the oil.