482 



Professor Silvanus P. Thompson 



[May 17, 



The effect of oblique resolution I may illustrate by introducing a 

 slice of tourmaline between the crossed Nicols, and rotating it till it 

 stands at 45^ ; or, in the acoustic model, by introducing an oblique 

 pair of guide pins. 



The other case — namely, that of producing a bodily twist of the 

 vibrations, rotating the plane of polarisation around the path of tlie 

 wave — is not so easily illustrated by the model. But it is optically 

 perfectly simple : all that is requisite is to introduce between the 



Fig. 



Ahrens's reflecting polariser. 



crossed Nicols a thin slice of that crystal — namely, quartz — in which 

 this effect of rotating the plane of polarisation was first observed. 



I take a clear plate of quartz, just 1 millimetre in thickness, and 

 interpose it between the crossed Nicol prisms. You will note how 

 the introduction of this plate of quartz brings some light into view. 



Suppose we now turn the analyser to try and obtain extinction : 

 we get tinting. If we put in a coloured glass so as to work with one 

 kind of light only, we shall get extinction at a particular angle. The 



