TO THE MICROSCOPE. 269 



slightly different rates, a complete cycle of changes taking place 

 when the gain of the quicker component ray amounts to a whole 

 vibration. Now, the thickness necessary for this cycle varies for 

 various colours ; consequently, the rays of different colours, after 

 passing through the section, are differently polarised, according to 

 the types of vibration represented in Figs. 5 (a — k). These rays 

 behave differently when passed through the analyser, and therefore 

 the section appears coloured. The colours of selenite are not 

 pure, but are formed of a mixture of all those component colours 

 which the analyser transmits. 



A few simple numerical examples will make this clear. 



J^irst. — Let us take the frequencies of vibration to be 400 

 billion vibrations a second in red, 600 billion in green, and 800 

 billion in violet light. Let us suppose the thickness of a film of 

 selenite to be such that the " extraordinary " ray takes one 800- 

 billiofith of a second less time to pass through it than the ordinary 

 ray. This gain of time in the extraordinary ray amounts to a 

 whole vibration for the violet light or half a vibration for the red 

 light. Consequently, the violet on emerging has exactly under- 

 gone its complete cycle of changes, while the red has only under- 

 gone half its cycle. The violet comes out polarised as at {k), 

 Fig. 5, and the red as at {e). With the Nicol's prisms crossed, 

 the red light is all transmitted and the violet all quenched ; with 

 the Nicols parallel, the red is quenched and the violet transmitted. 



The green light emerges in an intermediate stage — viz., circu- 

 larly polarised as at {g). It is partly transmitted whether the 

 Nicols are crossed or parallel. Of other intermediate colours, 

 those near the violet end — viz., blue and indigo — are transmitted 

 with greater intensity with the Nicol's prisms parallel than with 

 them crossed ; while those near the red end — viz., yellow and 

 orange— are transmitted with greater intensity wnth the prisms 

 crossed than with them parallel. 



The effect on the whole, then, is to give the selenite a blue 

 colour for one position of the prisms and an orange colour for the 

 opposite position. Thus the colours in a film of " blue and 

 yellow " selenite are accounted for. 



Second. — Suppose the thickness of the film doubled so that the 

 extraordinary ray in passing through gains one 400-binionfh of a 



