THE SIZE OF A TOMS. 1 8 1 



denser medium. The more refractive medium 

 (not always the denser) of the two has the less 

 velocity for light transmitted through it. The 

 " refractive index " of any transparent medium is 

 the ratio of the velocity of propagation in the 



FIG. 35. Diagram of Huyghen's construction for wave front of refracted light ; 

 drawn for light passing from air to flint glass. 



ether to the velocity of propagation in the trans- 

 parent substance. 



Now that the velocity of the propagation of 

 light should be different in different mediums, 

 and should in most cases be smaller in the denser 

 than in the less dense medium, is quite what we 



