I have told you already how we either absorb or reflect the æther waves which happen to fall upon us. In most substances it is only those electrons very near the surface that are disturbed. They succeed in stopping the waves. They may do this in either of two different ways. If the satellite electrons are attracted strongly by their atoms, the electrons will spin around the atoms keeping time to the movements of the incoming waves, and in this way the electrons take up the energy of the waves. In doing this, the electrons send out fresh waves in the æther. This is the real explanation of what man calls _reflection_ of light. [Illustration: THE SPECTROSCOPE AND THE ELECTRONS' WIRELESS MESSAGES The spectroscope is seen in the extreme left of No. 1 photograph. The instrument is explained at page 207. The operator is passing an electric current through a glass tube containing a rarefied gas, causing the gas to become luminous. When he examines its light through the spectroscope he sees bright lines as shown in photograph No. 2, and from the position of these lines he can tell what substance is producing the light. No. 2 is the spectrum of mercury vapour. No. 3 is part of the spectrum of the sun. Note the dark lines, as explained in the text.] In the second case, the electrons are not so firmly attached to their atoms, so that the incoming waves dislodge them, and they are knocked about from atom to atom, and in this way the energy of the waves is frittered away. Man speaks of the light having been _absorbed_ by the substance upon which it fell. In both cases the only electrons which take part in these actions are those electrons who can move in sympathy with the incoming waves. It will be clear to you that only those of us who are near the surface of a substance know anything about these incoming waves. The electrons attached to atoms in the interior of the substance are left in peace, owing to the defensive actions of our fellows on the outside. But this is not the case with all substances. There are some congregations of atoms through which the æther waves can make their way. Man calls such materials _transparent_; for example, glass and water are transparent substances. The fact of the matter is that in such substances none of us are able to respond to the incoming waves, and so we cannot stop them. I should say almost none of us, for there are always a few electrons present who happen to be in sympathy with the incoming waves. That is why no substance is perfectly transparent.