ON LIGHT. 385 



the analyzing plate), and differing in phase, will inter- 

 fere and give rise to the phenomena of coloration in tlie 

 manner already sufficiently explained. It remains now 

 to account for the colours being arranged in regular 

 succession in rings round the centre of the black cross 

 (which corresponds to the axis of the crystal). Now the 

 colour developed, or the order of the tint^ in the series of 

 the Newtonian rings, increases with the difference of 

 phase, and this difference increases with tlie difference of 

 velocities of the two pencils within the crystal, and with 

 the length of the path traversed with those velocities. 

 Both these increase with the inclination of the visual 

 ray to the axis of the crystal : since along the axis there 

 is no double refraction, which increases gradually from 

 that direction outwards up to a right angle. This, then, 

 explains the progressive increase of colour or order of 

 tint in proceeding from the centre outwards. The 

 circular arrangement is a consequence of the symmetry 

 of the crystalline plate in all directions around its axis ; 

 the amount of double refraction being the same at 

 equal obliquities to that line in all directions around it, 

 as also the increase of thickness traversed by rays 

 equally oblique in all directions to the surfaces of the 

 plate. It only now remains to explain how it happens 

 that, in this situation of the analyzing plate (at right 

 angles to the polarizing one), the tints are those of the 

 reflected^ not of the transmitted series in the Newtonian 

 rings. And the reason is very similar to that by which, 

 in the colours of tliin plates, the difference of phase is 

 assumed (justifiably assumed) to commence, not from 



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