222 PHYSICAL SCIENCE 



indeed, had long been imagined by philosophers, 

 as a means of transmitting actions from one body 

 to another, but its use as a physical explanation 

 of the phenomena of light first indicated some of 

 its necessary properties. The reflection of light 

 from the surface of a glass plate, or its passage 

 through certain doubly refracting crystals, such 

 as tourmaline, modifies the light, which acquires 

 properties not the same on all sides of the emergent 

 beam, and is then said to be polarised. No wave 

 system in which the direction of vibration is in 

 the direction of propagation can show such differ- 

 ences, for in such a system the waves must be 

 alike on all sides of their path. It follows that 

 the luminous vibrations must be transverse to 

 the direction in which the rays are travelling. 

 Transverse waves, if we are to regard them as 

 mechanical motion in a real medium with ordinary 

 dynamical properties, imply a certain amount of 

 rigidity or elasticity of shape in the medium — 

 such elasticity as is possessed by solids alone. 

 No fiuid when distorted has any tendency to 

 return to its original form ; it cannot transmit 

 waves which depend on mere distortional dis- 

 placements. Waves in a fluid must be waves of 

 compression and expansion, in which the direction 

 of vibration is in the direction of propagation. 



If, then, it is to carry a transverse wave-motion 

 of an ordinary mechanical kind, the luminiferous 

 aether must possess some of the properties of a 

 solid, and at one time the great problem of ^ethereal 

 physics consisted in formulating a medium possess- 

 ing the necessary rigidity. Any elastic jelly theory 

 leads to obvious difficulties when the passage of 

 matter through the aether is considered, a passage 



