to the Wave Theory of Light. 45 



If the surface of refraction should happen to have a node 

 V, which is a point of intersection where it admits an infinite 

 lumber of tangent planes (3), let the direction of the incident 

 ay S'OS be chosen, so that the right line US perpendicular 

 o the face of the crystal, being produced below 8, may pass 

 hrough JV, and we shall have a cone of refracted rays formed 

 >y the perpendiculars let fall from upon the tangent planes 

 ^ N ; all of which rays, on emerging parallel to OS from the 

 econd surface of the crystal, will be in complete accordance 

 rith one another. For we have just seen that if the ray S'OS 

 vere supposed to emerge after being refracted in the ordinary 



ON 



ray with an index equal to -y^, it would be in complete 



Accordance with any ray of the cone. 



48. The interval between any two rays emerging at the 

 ame side of the crystal is the difference of their retardations. 

 n taking the difference, the letters that are common to the 

 tames of the two rays may be left out. Thus the ray SPmMS 

 s behind the ray 8P8 by the interval 



r\ 



Che line - Pp is the interval between the rays SMS and 



, or between the reflected ray Os and the ray SPps, 

 Jid so on. 



49. The retardations of the two refracted rays SPS and 



Q 

 3MS, emerging without internal reflection, are ^ SP and 



^ SM respectively. The difference of these is 6 -^ . Con- 

 yo Oo 



equently, when the two refracted rays have emerged from the 

 econd surface in directions parallel to the incident ray, the 

 ight in the plus emergent ray is behind the light in the minus 



smergent ray by an interval equal to -^ . Or, in other 



C/o 



vords, the incident plane wave, perpendicular to OS, produces 



