268 POLARIZED LIGHT AND ITS APPLICATIONS 



means of the wave theory, let us for a moment consider what 

 constitutes white light. If a small pencil of sunlight is passed 

 through the prism of a spectroscope, we get a band of different 

 colours, called the spectrum^ and if a second prism is so placed as 

 to counteract the separating effect of the first, these colours will 

 be re-combined and will again form white light. The order of the 

 colours in the spectrum is : — Violet^ ifidigo, blue, greeji, yellow^ 

 orange red. Now, in the wave theory, these different colours 

 represent different rates of vibration of the ether, just as the dif- 

 ferent notes of the musical scale depend on the rates of vibration 

 of the sound-waves which strike on our ear. The slowest vibrations 

 which produce any impression of light on the retina of the eye 

 belong to the extreme red end of the spectrum, and their fre- 

 quency is jgg billion vibrations in a second. The quickest vibra- 

 tions visible in the form of light belong to the extreme violet end 

 of the spectrum, and of these 8ji billion strike the eye in a second. 

 Thus, the rates of vibration in the extreme violet and red rays 

 are in the proportion of about tivo to one — the same relation that 

 holds between a note and its octave in music ; in other words, our 

 range of vision extends over about an octave. Roughly speaking, 

 the velocity of light is the same for all colours, and as a conse- 

 quence the wave length or distance from one wave to the next in a 

 ray of red light is about double the wave length in violet light. 

 Fig. 6 shows the proportions of the wave lengths in red, green, and 

 violet light, enormously magnified of course, for when the light is 

 travelling in air there are really about '^2>->'^(^^ such waves of red, 

 43,197 of soda yellow, and 70,555 of violet light in the space of 

 a single inch. 



The colours of the spectrum are pure, but most ordinary 

 colours contain a certain mixture of these pure colours. And from 

 what has just been said, we see that ordinary white light consists 

 of a mixture of all the colours of the spectrum. 



Effect of Colour on Polariscopic Appearances.— In the last 

 section we showed that there is a certain periodicity in the changes 

 of type that take place in a beam polarised at an oblique angle 

 with the optic axes of a doubly-refracting section through which it 

 is passing. And these changes were shown to depend on the 

 ordinary and extraordinary rays travelling through the section at 



