PHYSICS OF THE NINETEENTH CENTURY. LIGHT. 471 



the results are reversed, the extraordinary ray is completely quenched, 

 and the ordinary ray passes freely. In the hands of an ingenious opti- 

 cian named Nicol, the polarizing property of Iceland spar was made 

 available as a far better means of studying the phenomena than that 

 afforded by the tourmaline. By availing himself of the difference of 

 the refractive indices of the two rays in the spar, he was able to turn 

 one of these rays out of the field, so that the properties of polarized 

 light might be studied in the other without intermixture of effects. 

 He accomplished this by taking a rhomb of the spar, and cutting it 

 obliquely in a certain direction, and after polishing the two cut surfaces, 

 cementing them together by a layer of Canada balsam. The refractive 

 index of Iceland spar for the ordinary ray is greater than that of Canada 

 balsam, but for the extraordinary ray it is less. Hence, if the ordi- 

 nary ray meets the Canada balsam at a sufficiently great angle of 

 incidence, total reflection oc- 

 curs. The section of the crystal 

 being made so as to obtain this 

 incidence, the ordinary ray is 

 reflected to one side, as shown 

 in Fig. 213, where it is seen 

 emerging from the crystal at o. 

 But the extraordinary ray, pas- 

 sing from a less to a more re- 

 fractive substance, cannot be 

 totally reflecte'd, and it is in 

 great part transmitted, emerg- 

 ing from the rhomb, as shown 

 at E, completely polarized. 

 The rhomb of Iceland spar thus 

 treated is known as " Nicol's 

 Prism." and its invention en- 

 abled many beautiful chromatic 

 effects of polarized light to be fully studied, as Iceland spar is itself 

 colourless and perfectly transparent. 



When the light polarized by reflection from transparent substances 

 is examined by Iceland spar (Malus' discovery, p. 452), or by a tour- 

 maline, or by Nicol's prism, the phenomena observed correspond with 

 those already described. For instance, if we place a plate of glass in 

 an upright position, and so that a beam of light falls upon it at an 

 angle of 32 with the surface, on interposing a Nichol's prism in the 

 path of the reflected beam between the eye and the glass plate we 

 shall find that in certain positions of the crystals the reflected beam is 

 wholly quenched, while in positions 90 to the former the light is 

 wholly transmitted. In the case supposed the beam will pass the tour- 

 maline when the axis of the crystal is vertical ; but when the axis is 

 horizontal it will be completely extinguished. Conversely, when the 



FIG. 213. 



