XI 



LIGHT 



177 



In this experiment the light is said to be dispersed owing to 

 the different refrangibility of the various kinds of light. An 

 examination of the band of colour or spectrum will show that 

 one colour shades imperceptibly into the next. There is, then, 

 an infinite number of waves of different lengths comprised in 

 the white light from the sun, and each ray is bent to an extent 

 depending on its wave-length. 



If the decomposed sunlight, instead of being collected on a 

 screen, be passed through a second prism similarly arranged, it 

 will be seen that the band is longer or the dispersion is greater. 

 The amount of dispersion also depends upon the material of 

 which the prism is made. Glass produces a much greater 



Fio. 90. The Analysis of Light by a Prism. 



amount of dispersion than water ; flint-glass possesses twice the 

 dispersive power of crown glass ; carbon bisulphide, again, has 

 more dispersive power even than flint-glass. 



Although a continuous band of colour is observed when sun- 

 light, or limelight, or a gas or candle flame is seen through a 

 prism, this continuous spectrum is not always produced. If 

 when substances such as sodium, strontium, and lithium, or 

 their compounds, are burnt in a non-luminous flame, and ob- 

 served through a prism a discontinuous spectrum is seen. The 

 following experiment, described by Sir Norman Lockyer, illus- 

 trates this. 



