8o GREAT MEN OF SCIENCE 



this case, he himself supplies an entirely new foundation 

 based on observation, and supported by entirely new argu- 

 ments. What he found as known, were the rectilinear pro- 

 pagation and reflection of light, understood since Euclid's 

 time; the refraction of light, studied by Kepler and, more 

 accurately by Snell, together with its successful application to 

 the telescope by Galileo; and finally, and of particular 

 importance, the propagation from the source to the eye with 

 a limited and known velocity, determined by Roemer only 

 a short time previously. Huygens combined these and all 

 other known facts concerning light and its production, and 

 concluded that we must suppose it to consist of a vibration 

 which spreads outwards from the source, just like sound in air. 

 The medium, however, in which the light is propagated as 

 vibration, cannot be the air. Huygens deduces this from 

 Guericke's, and later Boyle's observation of the propagation 

 of light through a space empty of air, and then draws the 

 further conclusion that space free from all matter, even air, 

 must still contain a something capable of permitting vibra- 

 tion to be transmitted. This something is called by Huy- 

 gens the ether. With him, therefore, we have the beginning 

 of the physics of the ether. The chief point, however, is 

 the proof that light actually, and in every respect, possesses 

 the peculiar properties of a state of vibration which is 

 transmitted in waves, and here Huygens contributed the 

 first pieces of evidence which have been of importance 

 ever since. He first of all shows how the property of light 

 waves, coming from any direction, of penetrating one 

 another without hindrance or disturbance, is precisely what 

 we should expect from the propagation of vibrations; it is 

 found also in the case of sound, and, as he showed, in the 

 collision of elastic bodies. Here he makes the observation, 

 founded on experiment, that balls in collision experience a 

 momentary flattening, a process obviously requiring time, 

 so that it is clear that a blow transmitted through a row of 



