406 



PHYSIOLOGICAL PHYSICS. [Chap. xxx. 



x*' 



angles to one another because the Nicols are crossed, 

 and, consequently, the ray transmitted by the first 

 will be extinguished by the second. Let Pt Pt 

 represent the principal plane of the doubly refractive 



plate. The extraordinary 

 / ray transmitted by the first 

 Nicol vibrates parallel 

 to the plane NN 1 , and, since 

 it falls obliquely on the 

 ~ N2 plate, it is split into two 

 rays, an extraordinary and 

 an ordinary at right angles 

 to one another, i.e. one 

 vibrating in the plane Pt, 

 Fig. 184. Polarisation of Light, arid another in the plane Pt r . 



These two rays meet the 



second Nicol, but it can only transmit vibrations 

 in the plane N 2 . The vibrations in Pt can, however, 

 be resolved into a vibration in N 1 and a vibration in 

 N 2 (see page 40 1 ) ; the former is extinguished, the latter 

 transmitted. Similarly the vibration in pf can be 

 resolved into a vibration in N 1 and a vibration in N 2 , 

 the former being extinguished, and the latter 

 transmitted. Thus by the position of the doubly 

 refractive plate the crossed Nicols become illumi- 

 nated, the illumination being due to two sub-rays, one 

 a sub-ray of the vibration in pt, and the other a sub-ray 

 of the vibration in Pt r , which have been made to 

 vibrate in N 2 . 



Interference. Another phenomenon makes its 

 appearance when the arrangement of two Nicols and 

 an interposed refractive plate is used, as just described, 

 a phenomenon not visible with a thick plate of 

 Iceland spar, but seen when a very thin plate is used 

 or a thin lamina of selenite (crystallised gypsum). It 

 consists in the appearance of colours varying accord- 

 ing to the position of the Nicols. They are brightest 



