IV.] 



SUNSET COLOURS. 



73 



commencing in the red, and gradually extending over everything 

 but the violet. 4, 5, and 6 represent the effect produced by 

 various thicknesses of a blue glass with a tinge of purple, 4 

 showing the action of the least thickness, and 6 that of the 

 greatest. 



It will be seen from these figures that Herschel observed that 

 this stoppage of light affected not only large reaches of the 

 spectrum, but as shown in Fig. 16 (4), it was sometimes so closely 

 localised as to produce sharp lines. He was the first to point out 

 this fact clearly. These phenomena are produced by the 

 absorption of light by the various bodies interposed in the path 

 of the beam on its way to the eye. We shall have to dwell on 

 this effect at considerable length in the sequel. 



Such experiments as these explain the phenomena of sunset 

 colours, which deserve notice here because, as it will be seen 

 later on, the work done by our atmosphere on the sunlight 

 requires to be completely studied before we can get any certain 

 knowledge of the light actually emitted by the sun. 



One form of the aqueous vapour in our atmosphere exerts 

 a powerful obliterating action on the solar light, commencing 

 at the blue end, and gradually encroaching on other parts of 

 the spectrum, thus : 



{TV No absorption. 



v a [ v 



V I [IB Y 



V I B 



V I B G 



VI BGY 

 VI BGYO 



Showing the 



; effect of increasing 



absorption. 



