402 PHYSIOLOGICAL PHYSICS. [Chap. xxx. 



another. Thus the phenomena witnessed in Iceland 

 spar are due to the light being polarised, both the 

 extraordinary and the ordinary ray being polarised, 

 the one at right angles to the other. 



Simply refractive bodies do not possess the pro- 

 perties of splitting up natural light in this way, at 

 least to the same extent. They are called isotropous, 

 while doubly refractive bodies are called anisotropous. 



It is to be noticed that in the polarised ray which 

 emerges from a Nicol's prism there is nothing to render 

 its peculiar condition appreciable by the unaided eye ; 

 but as soon as the eye is aided by a second Nicol's 

 prism, the condition is recognised by the fact that on 

 rotating the prism the beam of light from the first 

 prism is extinguished, and reappears on continuing 

 the rotation. The condition produced by the first 

 prism is only recognisable by the aid of a second or a 

 similar doubly refractive body. The second is, there- 

 fore, called the analyser, while the first is called the 

 polariser. The explanation of the alternate darkness 

 and light produced by rotation of one prism on 

 another is, that the ray which emerges from the first 

 prism will be transmitted by the other so long as the 

 principal planes of the two prisms are parallel, and 

 will not be transmitted at all when the planes are at 

 right angles to one another. In two positions the 

 planes are parallel, and in two at right angles. Sup- 

 pose they are parallel at first, the light is bright ; on 

 rotating one through an angle of 90 they are at right 

 angles and the ray is extinguished. If the rotation 

 be carried on to 180 the. planes are again parallel 

 and again the light is bright ; but on passing through 

 another 90 they are again at right angles and the 

 light is again extinguished. A further quarter turn 

 brings the prisms back to their original parallel con- 

 pition. With many other crystals similar phenomena 

 may be exhibited. 



