POLARISED LIGHT. 7 



light. In the case of transparent substances there is a very 

 Simple law, as follows : When light falls upon the surface of 

 a transparent medium part is in general reflected and part, 

 after undergoing refraction or bending, is transmitted. The 

 angle of reflexion is, as you well know, equal to that of inci- 

 dence ; that of refraction depends upon the relative densities 

 of air and of the transparent medium ; and the angle between 

 the reflected and the refracted rays will therefore vary with 

 the angle of incidence, and when the latter angle is such 

 that the reflected and refracted rays are at right angles to one 

 another, the light, both reflected and refracted, is most com- 

 pletely polarised. This angle of incidence, which is of course 

 different for different substances, is on this account called the 

 polarising angle. You will doubtless remember that the 

 polarisation of the reflected ray is perpendicular to that of 

 the refracted ray. 



Beside the method which we have as yet used, viz. re- 

 flexion and refraction by glass, there are other methods of 

 polarising and analysing light. The most important of these, 

 and the one which I now propose for your attention, is the 

 transmission through crystals. Generally speaking crystalline 

 bodies reflect and refract light like glass, or water. But, 

 unlike glass or water, they divide every ray which they 

 transmit into two. Here is a block of Iceland spar, or 

 calcite, the crystal generally used in researches on polarised 

 light ; first, because it is perfectly colourless and transparent ; 

 secondly, because it is found in masses sufficiently large for 

 all optical purposes ; and thirdly, because it possesses the power 

 of separating the rays into two in a very high degree. Its 

 natural form is that of a rhombohedron, having two obtuse 

 and two acute angles. Eays of light passing through such a 

 crystal are, as observed before, generally divided into two, or 

 doubly refracted. But there is one direction, and one only, 

 viz. that of a line joining the two obtuse angles, in which no 

 such division takes place. This line is called the axis of 

 the crystal. The greater the angle made by the incident ray 

 with the axis, the greater the separation ; and at right angles 

 to the axis the separation is greatest. The block of spar 

 which I hold in my hand has had its two blunt angles cut 

 off; and if we look through the faces so artificially formed, 

 we shall see a single image of objects beyond ; but if we look 

 through in any other direction we shall see two images. 



