410 Professor Aiky on the Phenomena 



a theoretical calculation of the intensity of light in the rings : 

 as without this the necessity for some of the precautions will not 

 be sufficiently evident. 



Conceive two nearly parallel plates of different media to be 

 separated by a plate of air whose thickness is T; and let the 

 vibration in the plane of reflexion, of an incident stream of 



light within the first medium, be represented by «sin — - (vt-x) 



A 



where x is the equivalent in air to the actual distance of a par- 

 ticle from some fixed point, (the light being supposed polarized 

 in a plane perpendicular to the plane of reflexion). Let < be the 

 angle of incidence on the last surface of the first medium ; i the 

 angle of refraction, which is the same as the angle of incidence 

 on the first surface of the second medium; and <" the angle of 

 refraction in the second medium. A part of the light will be 

 reflected at the last surface of the first medium : a part will reach 

 the first surface of the second medium, where it will be sub- 

 divided ; and one portion will be reflected to the surface of the 

 first medium where it will be again divided, and one of its parts 

 will enter in the same direction as that which was reflected at 

 first. In this the phase of the undulation will be behind that which 

 was first reflected by the quantity corresponding to the space 

 SlTcosi': or if —(vt—x) be still taken as the measure of the phase 



of the ray first reflected, -^(vt-x) - -^ Tcosl will be that of the 



A A 



ray which has been reflected at the surface of the second medium 

 and then enters the first. The quantity — -Tcost we shall for 



A 



abbreviation call V. Of the light which reaches the surface of 

 the first medium, a part will be partially reflected at the surface 



