158 POPULAR LECTURES AND ADDRESSES. 



the wooden rods represent pieces of matter I 

 will not say molecules at present, though we 

 shall deal with them as molecules afterwards. 

 Light consists of vibrations transverse to the 

 line of propagation, just as in the models before 

 you. 



Now in that beautiful experiment well known 

 as Newton's rings we have at once a measure of 

 wave-length in the distance between two pieces 

 of glass to give any particular tint of colour. 

 The wave-length, you see, is the distance from 

 crest to crest of the waves travelling up the long 

 model when I commence giving a simple har- 

 monic oscillation to the lowest bar. I have here 

 a convex lens of very long focus, and a piece of 

 plate glass with its back blackened. When I press 

 the convex lens against the piece of plate glass 



liquid (treacle diluted with water) contained in the vessel (C). A 

 heavy weight resting on the bottom of this vessel, and connected to 

 the lower end of the suspending wire by a stretched indiarubber 

 band, serves to keep the lower end of the apparatus in position. 

 The period of vibration of the pendulum bar is adjustable to any 

 desired magnitude by shifting in or out the attached weights, or by 

 tightening or relaxing the cords which pull the upper ends of the 

 spiral springs. 



