4-20 Professor Airy on the Phenomena 



this takes place at an angle where (so far as we are entitled 

 to conclude) there is nothing peculiar in the reflexion from the 



"lass; and we are therefore compelled to admit that, the incident 



2?r ... 



vibration being a . sin — (vt-x), when the angle of incidence is 



increased so as to exceed that angle the reflected vibration is 



(■hanged from + />.sin — (vt—x) to-^.sin — - (vt—x). A similar 



A A 



change takes place at the polarizing angle of the glass: but 

 there, as we have seen, the transition from + p to - q is effected 

 by passing through 0, or by the entire cessation of reflection 

 at one angle of incidence ; which is not the case at the po- 

 larizing angle of the diamond. How then is the gradual change 



from + p sin — (vt — x) to — q . sin — (vt — x) to be explained? I 



A. A 



answer that the phoenomena prove that it follows from a gradual 

 change of phase, while the coefficient is not much altered. In 

 other words (neglecting the trifling alteration in the coefficient) 



the quantity +osin — (vt—x) is changed to — p sin — (vt—x), not 



A A 



by the disappearance of p, but by the expression assuming the 



form p sin |~ (vt—x) — 6>, where increases from o to v, This 



may be popularly explained in the following mariner. The 

 common Newton's rings, formed between two lenses, are produced 

 by the interference of the light reflected from the lower surface 

 of the upper lens with that reflected from the upper surface 

 of the lower lens. Now if the upper lens be raised a little, or 

 the lower depressed a little, the rings contract. As the only 

 immediate effect of depressing the lower lens is to cause the 

 light reflected from it to describe a longer path, or to have its 

 phases retarded, it appears that a contraction of the rings may 



