POLARISED LIGHT. 11 



lengths, or of various refrangibilities. All the waves of which 

 that white light is compounded are retarded through the same 

 absolute distance, the same fraction of an inch, by its passage 

 through the crystal plate, and therefore the waves of different 

 lengths will have been retarded through different fractions 

 of the wave-length. In other words, the distance through 

 which they are retarded being the same for all wave-lengths, 

 will be a larger fraction of the short waves than of the long 

 waves. The amount of retardation depends, as before observed, 

 on the thickness of the crystal j and the thickness may be 

 such as to produce a retardation which, for some particular 

 wave, is exactly half a wave-length. If so, that particular set 

 of waves will, as the machine showed us just now, be anni- 

 hilated or extinguished. By this process, therefore, from the 

 white light one particular colour will have been subtracted, 

 and there will remain an assemblage of colours which 

 being incapable of reproducing white light will combine to 

 form a residual tint. It is of great importance to realise this 

 process. To recapitulate, if between the polariser and analyser 

 there be introduced such a crystal, we shall not only have the 

 effect I have described in the first instance, of extinguishing 

 the light, but when the analyser is in the proper position, we 

 have colour produced by the extinction of a certain portion 

 or portions of white light. There are different kinds of 

 crystal which for the same thicknesses produce different effects, 

 but for the same crystal it is easily seen that the amount of 

 retardation will depend on the thickness of the plate, because 

 the retarding action of the substance through which the 

 divided rays pass, is in operation during the entire passage. 

 If, therefore, a crystal of one thickness extinguishes one set 

 of waves, a thicker plate will extinguish a longer set, and 

 generally different thicknesses of any given crystal will extin- 

 guish different component portions of the white light, and 

 the resulting colours will also be different. In fact, by care- 

 fully selecting plates of suitable thicknesses we can pro- 

 duce in the field of view any arrangement of colours at 

 pleasure. 



The majority of crystals polarise light in the manner above 

 described ; but there is another kind of crystal which pro- 

 duces polarisation of a different kind. I said, in the outset, 

 that the vibrations might be either straight lines, or circles, 

 or ellipses ; but we have hitherto confined our attention to 



