Now imagine a row of little floats placed along the line EDCBABCDE. At the lettered points the floats will be violently agitated, but at the points midway between the letters they will be unmoved. This exactly represents the effect of two interfering sources of light S, S, sending light which is received by a screen at the dotted line EDCBABCDE. The lettered points will be brightly illuminated while the intermediate points will be dark. In practice it is found impossible to make two {16} sources of light whose vibrations start at exactly the same time and are exactly similar, but this difficulty is surmounted by using one source of light and splitting the waves from it into two portions which interfere. +Young's Experiment.+--Dr. Young's arrangement is diagrammatically represented in Fig. 7. Light of a certain wave length is admitted at a narrow slit S, and is intercepted by a screen in which there are two narrow slits A and B parallel to the first one. [Illustration: FIG. 7.] A screen receives the light emerging from the two slits. If the old corpuscular theory were true there would be two bright bands of light, the one at P and the other at Q, but instead Dr. Young observed a whole series of parallel bright bands with dark spaces in between them. Evidently the two small fractions of the original waves which pass through A and B spread out from A and B and interfere just as if they were independent sources like the two styles in the mercury ripples experiment. {17} +Speed of Light in Rare and Dense Media.+--The discovery of interference again brought the wave theory into prominence, and in 1850 the death-blow was given to the corpuscular theory by Foucault, who showed that light travels more slowly in a dense medium such as glass or water than in a light medium such as air. This is what the wave theory anticipates, while the reverse is anticipated by the corpuscular theory. But if light and heat consist of waves, what kind of waves are they and how are they produced? +Elastic Solid Theory.+--In the earlier days of the wave theory it was supposed that the whole of space was filled with something which acted like an elastic solid material in which the vibrations of the atoms of a luminous body started waves in all directions, just as the vibrations of a marble embedded in a jelly would send out waves through the jelly. These waves are quite easily imagined in the following way.