PHYSICAL FORCES 113 



If the + action of one ray coincides with the + of the 

 other ray (both being of equal intensity) the visible effect 

 will be doubled ; but if the + action of one synchronises 

 with the - action of the other, the effect of each will be 

 neutralised and produce darkness, or a case of interference 

 of light. Similarly, if white light falls upon any body 

 the surface of which is minutely varied or which contains 

 two or more reflecting surfaces, the rays, the steps of 

 which are of different lengths, will be variously reflected 

 or transmitted ; and so the white light will break into 

 colours and we shall have the phenomenon known as 

 iridescence. Thus it is that, (i) mother of pearl (made 

 up of extremely thin laminse super-imposed), or (2) a 

 surface marked with parallel grooves exceedingly close 

 together, or (3) two glass plates with a very thin film of 

 air between them, or (4) the very thin film constituting 

 a soap bubble, will each and all appear iridescent. 



The last phenomenon of light to which we deem it 

 here necessary to refer, is that termed polarisation. 

 Light coming from the sun, or from any incandescent, 

 self-luminous body, can be reflected or refracted equally 

 well (equally brightly) in all directions, according to the 

 movement of rotation which may be imparted to the 

 body which reflects or refracts it. This, however, is not 

 the case with light which does not come from a self- 

 luminous body but from one which merely reflects light. 

 Such reflected light, when a movement of rotation is im- 

 parted to the body which reflects or refracts it, cannot be 

 reflected or refracted equally well in all directions, but will 

 be more intense when sent in some directions than in 

 others. Such light may be said to have acquired " sides," 

 or some property which facilitates its energy in some direc- 

 tions and restrains it in others. This property is called 

 polarity, and (as we might now expect from what has been 



