408 PHYSIOLOGICAL PHYSICS, [Chap. xxx. 



another. Now, with the plate of selenite as 

 described, we have seen that the light passes through 

 the crossed Nicols because it is decomposed into two 

 vibrations at right angles to one another, the ordinary 

 and extraordinary ray. We have seen, also, that the 

 illumination is due to two sub-rays, one of P (Fig. 

 184), and another of p r , which have been made to 

 vibrate in the same plane, one being a sub-ray of the 

 ordinary and the other of the extraordinary ray. But 

 though the sub-rays vibrate in the same plane they are 

 of different velocities, because of the difference between 

 the retardation experienced by the ordinary and extra- 

 ordinary ray in passing through the doubly refractive 

 plate. Hence the phases of the two vibrations do not 

 coincide, and thus they exhibit the phenomena of 

 interference. This implies the extinction of certain 

 rays of the white light, and the light that is seen 

 through the second Nicol will be white light less the 

 extinguished rays. The interference affects different 

 rays of white light according to the position of the 

 Nicol's prisms, but the rays that are extinguished and 

 the rays that are transmitted will together form white 

 light, and are thus complementary to one another. 



Coloured rings, due to interference, are observed 

 when a thin film, of transparent material separates 

 two media with refractive index different from its 

 own. Thus the colours of a soap bubble are due to 

 interference by the reflections from the surfaces of the 

 film in contact with air on each side. These rings of 

 colours are called Newton's rings, because Newton 

 first studied them carefully. 



The polariscope in physiology. " When 

 muscular fibres are examined with a microscope, to 

 which a polarising apparatus is attached, remarkable 

 and instructive phenomena are observed. If the field 

 be darkened by crossing the planes of polarisation of 

 the Nicol's prisms, those fibres only disappear which 



