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that colour can be produced by the mere motion of a white 
ray without the aid of refraction; his object now was to 
explain those physical laws by which colour is produced 
from a homogeneous substance. 
He argued that as we hear through the medium of the 
air — a homogeneous substance — and the variety of sounds 
is equal to the variety of colours, we may have as great a 
variety of colour from the ether considered as homogeneous 
as of sounds from the air so considered. 
The difficulty was to prove by what process nature modi- 
fies so wonderfully the motion of a homogeneous substance. 
Many years ago he found that by pressing a piece of glass 
with a point he obtained colours. If the glass pressed were 
laid on an irregular surface, as a piece of ivory or steel, the 
colours were in irregular patches ; but if the glass were laid 
on another piece of glass, the circles were complete. 
This proved that Newton’s rings, as they are called, were 
produced by undulations or waves in the glass, that these 
waves are virtually small prisms, and that they are stationary 
as long as the pressure is stationary. 
By and by he found that similar waves or bands exhibited 
themselves in a film of soap when made to cover the mouth 
of a tumbler, and that they were particularly interesting 
and instructive when the tumbler was held at an angle to 
the horizon. He also found that by blowing through a 
tube on a plate of glass, a converse series of rings was 
obtained. And as a proof that these rings were formed of 
waves, we had merely to examine their motions — the motion 
of the one series was the converse of the other. 
Again, he took a thin disc of glass and laid it on another 
piece of glass of any thickness, and subjecting it to pressure 
