270 POLARISED LIGHT AND ITS APPLICATIOXS 



second on the ordinary ray. This difference of time amounts to 

 2 vibrations for violet, ih for green, and i for red light; conse- 

 quently, on emerging, the violet has undergone exactly 2 cycles, 

 the green i|, and the red i complete cycle of changes. The 

 violet and red come out polarised as at (a) or (k), Fig. 5, and the 

 green as at (<?), With the Nicols' prisms crossed, we get green 

 light transmitted, but no red or violet ; with the Nicols parallel, 

 red and violet are transmitted and green is quenched. 



As before, the intermediate colours are affected in an inter- 

 mediate manner. We get on the whole purplish red for one 

 position of the prisms, and green for the opposite position. 'I'he 

 the colours in a film of " red and green " selenite are accounted 

 for. 



Third. — If the thickness of the selenite is now increased very 



slightly, the colours in which one ray gains \\ and 2 vibrations on 

 the other, will correspond to rather slower rates of vibration than 

 before. Instead of green we shall get yellow light with the 

 Nicols' prisms crossed, and instead of violet we shall get blue with 

 the prisms parallel. We here again have a film of blue and yellow 

 selenite, but the tints are not quite the same as they were before. 



Fourth. — For a much greater thickness a number of different 

 colours are transmitted when the prisms are crossed, and a number 

 of other colours are transmitted with the prisms parallel. The 

 resulting colours are more mixed than they were before, and are 

 consequently much less conspicuous. When the section is very 

 thick, this mixture gives rise to practically white light in every posi- 

 tion of the analyser, though if this white light is examined with a 

 spectroscope it will still be found that a number of colours are 

 missing in either position. 



Fifth. — For a very thin polarlscopic object we again get w^iite 

 light with a dark background. In all the colours the change of 

 type produced by the object is only sufficient to make the emerg- 

 ing light slightly elliptically polarised, the directions of the light- 

 vibrations being changed from straight lines to narrow ellipses, as 

 at Fig. 5 {b). The change has most effect on the quicker vibra- 

 tions, which are therefore most strongly transmitted by the 

 "crossed" analyser; but all colours are more or less transmitted. 

 Hence the object appears of the rather blueish-\s\\\\.Q. tint with 

 which we are so familiar. 



