For instance, interpose the very thinnest sheet of metal between a candle and the eye, and the light is cut off. But the sheet will very readily convey electricity. On the contrary, glass, a substance that repels electricity, is transparent, _i.e._ gives passage to light. And again, electricity can be conveyed round as many corners as you please, whereas light will travel in straight lines only. To clear away our doubts we have only to take the lighted candle and again hold up the metal screen. Light does not pass through, but heat does. Substitute for the metal a very thin tank filled with a solution of alum, and then light passes, but heat is cut off. So that heat and electricity _both_ penetrate what is impenetrable to light; while light forces a passage securely barred against both electricity and heat. And we must remember that open space conveys all alike from the sun to the earth. On meeting what we call solid matter, ether waves are influenced, not because ether is wanting in the solid matter, but because the presence of something else than ether affects the intervening ether itself. Consequently glass, to take an instance, so affects ether that a very rapid succession of waves (light) are able to continue their way through its interstices, whereas long electric waves are so hampered that they die out altogether. Metal on the other hand welcomes slow vibrations (_i.e._ long waves), but speedily kills the rapid shakes of light. In other words, _transparency_ is not confined to light alone. All bodies are transparent to some variety of rays, and many bodies to several varieties. It may perhaps even be proved that there is no such thing as absolute resistance, and that our inability to detect penetration is due to lack of sufficiently delicate instruments. The cardinal points to be remembered are these:-- That the ether is a universal medium, conveying all kinds and forms of energy. That these forms of energy differ only in their rates of vibration. That the rate of vibration determines what power of penetration the waves shall have through any given substance.