I have it in my power to show you a still more remarkable experiment in electric refraction. It is found that dry ice is very transparent to these electric rays, but if the ice is wetted on the surface, then, as you have already learnt, the film of moisture is opaque. We have had constructed for the purposes of this lecture a prism of ice by freezing water in a properly shaped zinc box. This prism is now being arranged between the radiator and the receiver, and its surfaces must next be dried carefully with dusters and white blotting paper to remove every trace of moisture. When this is done we find we can repeat with the ice prism the same experiment performed just now with the paraffin prism, and we can refract the electric ray. If you will recall to your memory the statements which were made in connection with the refraction of rays of sound and waves of water, you will remember that it was pointed out that the refraction of a ray of sound and the bending of a train of water waves was due to the passage of the waves in the air or in the water from a region where they were moving quickly to a region in which they were compelled to move more slowly; and it was furthermore shown that this bending must take place whenever a plain wave of any kind passes in an oblique direction from one region to another region where it undergoes an alteration in velocity. In other words, it was shown that the bending or refraction of the direction of motion of a wave, whether in air or water, is a proof that there is a difference in its velocity in the two places bounded by the surface at which the refraction takes place. If this bending takes place in such fashion that the ray is bent towards the perpendicular line drawn to the bounding surface, which is the same thing as saying if the line of the wave is bent so as to make a less angle with the bounding surface after it has passed from one region to the other, then it shows that the wave-motion travels more slowly after it has passed the bounding surface than before. If we now return to the consideration of the electric experiment with the prism of paraffin or ice, we shall find that this, properly interpreted, gives us a proof that the electric radiation travels more slowly in paraffin wax or ice than it does in air, and the ratio between its velocity in air or in empty space and its velocity in any non-conductor is called the _electric index of refraction_ for that non-conductor. This index can be determined by making two measurements. First, that of the refracting angle of the prism; and secondly, that of the deviation of the ray.[26] I have made these two experiments for the prisms of paraffin and ice in my laboratory, and I find the electric refractive index of paraffin to be 1·64, and the electric refractive index of ice to be 1·83.