182 POPULAR LECTURES AND ADDRESSES. 



should, according to dynamical principles, expect 

 from any conceivable constitution of the lumini- 

 ferous ether and of palpable transparent substance. 

 But that the velocity of propagation in any one 

 transparent substance should be different for 

 light of different colours, that is to say, of dif- 

 ferent periods of vibration, is not what we should 

 expect, and could not possibly be the fact if 

 the medium is homogeneous, without any limit 

 as to the smallness of the parts of which the 

 qualities are compared. The fact that the velocity 

 of propagation does depend on the period, gives 

 what I believe to be irrefragable proof that the 

 substance of palpable transparent matter, such as 

 water, or glass, or the bisulphide of carbon of 

 this prism, whose spectrum is before you, is not 

 infinitely homogeneous ; but that, on the contrary, 

 if contiguous portions of any such medium, any 

 medium in fact which can give the prismatic 

 colours, be examined at intervals not incom- 

 parably small in comparison with the wave- 

 lengths, utterly heterogeneous quality will be 

 discovered ; such heterogeneousness as that which 



