132 POLARISED LIGHT AND ITS APPLICATIONS 



Application to transmission of Light through Crystals. — 



Now, a precisely similar cycle of changes takes place, according 

 to the wave theory, when a ray of light falls on a section of 

 doubly-refracting material polarised in any direction other than 

 along one of the two optic axes. Suppose that on entering the 

 section the ray is polarised in the direction //'', inclined at 45^ 

 to the optic axes EE,, 00' . Then at the surface the ether is, of 

 course, vibrating in straight lines in the direction //'. As the 

 light penetrates into the section, the ether begins to vibrate in 

 ellipses, and the light is said to be elliptically polarised. At a 

 certain depth the light becomes circularly polarised. At double 

 this depth (Fig. 5 e) it becomes polarised along the diameter UU^ 

 perpendicular to the original direction of polarisation. At treble 

 the same depth it is again circularly polarised, but the ether is 

 revolving in the opposite direction to what it was before. At four 

 times that depth the light has gone through a complete cycle 

 of changes, and is polarised in the same way as it was on entering. 

 As the light penetrates further and further into the section, the 

 same cycle of changes keeps recurring over and over again. 



Now let the emergent light be examined with an analyser 

 placed perpendicularly to the polariser, in the position known as 

 " crossed." In this position it will transmit all vibratory motion 

 of the ether in the direction UU'^ but stop all vibration along //'. 

 If the thickness of the section be such that the light emerges 

 at the middle of the cycle of changes which we have just 

 described — i.e.^ in the stage represented at {e) — it will be polarised 

 along UU'^ and will be entirely transmitted by the analyser. The 

 section will therefore appear bright while the field of view is dark. 

 On the contrary, if the light happens to have undergone exactly 

 one, two, three, or more cycles of changes, when it comes out it 

 will be polarised as it was on entering, and the object as well as 

 the field will appear dark. Unless, however, the light comes out 

 exactly at the end of a cycle, it will be in a condition to be, at 

 any rate partially, transmitted by the analyser, and the object will 

 appear more or less brightly illuminated, and the perfectly dark 

 background will render it conspicuous. 



Suppose the analyser turned round parallel to the polariser, 

 giving a bright background. It now transmits light polarised along 



