POLARISED LIGHT. 9 



analyse by a Nicol prism, or vice versd. In fact, any in- 

 strument which will serve as a polariser will serve equally 

 well as an analyser. And by means of any one of these 

 instruments we can convert a beam of ordinary into a beam 

 of polarised light, whose plane of polarisation lies in any 

 required direction. 



Now out of this effect of double refraction another impor- 

 tant property of crystals follows as a consequence. When a 

 ray of light passing from a rarer to a denser medium, as from 

 air to glass or water, falls obliquely on the surface separating 

 the media, it is refracted or bent out of its original direction 

 towards (i.e. in such a way as to make a smaller angle than 

 before with) the perpendicular drawn at the point of incidence 

 to the surface. This refraction or deviation is due to the 

 slower rate at which the light travels in the denser than in 

 the rarer medium. Hence we may conclude that of the two 

 rays produced by double refraction that which is most deviated 

 moves slowest. The entire action of the crystal is conse- 

 quently, first, to divide each ray into two, or doubly refract it ; 

 secondly, to polarise both these rays in planes at right angles 

 to one another ; and thirdly, to make one of the rays to tra- 

 verse the crystal slower than the other, or to retard one more 

 than the other. If, therefore, we consider, as we must, that 

 the two sets of rays consist of waves of the same length, then 

 the waves of the one set will be retarded, or made to lag 

 behind the other to a certain extent; and if by some 

 mechanical or optical arrangement we bring these two sets of 

 vibrations, which are now at right angles to one another, 

 into one and the same plane after they be emerged, then 

 instead of the waves being exactly coincident as they would 

 have been if the light had moved all with the same velocity 

 through the crystal, the one will be somewhat behind the 

 other ; that is, the crests of one set of waves will be behind 

 the crests of the other set, and the hollows of the one set be- 

 hind the hollows of the other. Although the result of this 

 is pretty well known, I will, nevertheless, bring to your 

 notice a little instrument devised by the late Sir C. Wheat- 

 stone, by which the effect is very clearly shown. As now 

 arranged, the two sets of waves coincide ; but by moving a 

 slide so as to bring the crests of one set to coincide with the 

 hollows of the other, the effect is to reduce the combined wave 

 to a straight line, whereas, when the crests are coincident the 



