205 Re/lection and Refraction of Light, 



In conclusion let it be observed, that this theory may be applied with 

 equal ease to the explanation of the important, complicated, and brilliant 

 phasnomena of thin and thick plates, diffraction, double refraction, and 

 polarisation of light, plain and circular, with the phaenomena of coloured 

 rings by polarised light, &c. as may be seen in the Physical Treatise of 

 Optics, to which reference has been made. 



Probably the following observations, relative to the polarisation of light 

 may be acceptable to the reader ; let B A, fig. 4, be the direction of the 

 ray, orpq & plane at right angles to itj the transverse pulse will be in 

 this plane : first let it be in the direction op, in the plane of incidence; 

 at the moment when B arrives at the surface A F, if the transverse pulses 

 be moving in the direction op, that is towards the surface, it will favour 

 reflection by pressing on the ethereal matter at the common surface ; but 

 if it be returning in the direction p o, it will favour transmission, by taking 

 off a portion of the pressure, and since it will be as often in one of these 

 positions as in the other, the differences in the quantity of light reflected, 

 or transmitted, will not be altered in consequence of this pulse. 



Again, let the transverse pulse be perpendicular to op, that is at right 

 angles to the plane of incidence, and consequently parallel to the surface of 

 the new medium : when the atom of light B arrives at A, the accumulated 

 ethereal matter at the surface will be affected evidently by this pulse, in 

 the same manner, whether it is going or returning ; and in both eases it is 

 throwing ethereal matter over the centre of the moving atom, and there- 

 fore always favours transmission to which independently it has a tendency ; 

 hence in this position of the pulse, it will be always transmitted, and the 

 transverse pulse will continue in the same direction, since there is nothing 

 to produce a change in this respect. It will be seen from what is advanced, 

 that at any azimuth between the two former, more light will be transmitted, 

 and consequently less reflected, as the transverse pulse is nearer to the 

 last-mentioned position. 



Besides this, when the transverse pulse is neither in the plane of inci- 

 dence, nor at right angles to it, the resistance at the surface will bring it 

 nearer to one or other of these planes, according as it is reflected or trans- 

 mitted, which is easy to prove, so that the reflected light will have the 

 several transverse pulses nearer to a coincidence with the plane of reflec- 

 tion, and that of the transmitted light nearer to a perpendicular to that 

 plane ; and this circumstance unfolds at once the hitherto-mysterious cause 

 of polarisation in all its peculiarities. 



Sir J. Herschel observes, that the phaenomena of diffraction " are com- 

 pletely accountable for on the undulatory doctrine, and form, in fact, its 

 strongest points." Now the explanation is derived from the doctrine of 

 the interference of light ; the fringes being supposed to be produced by 

 the interference of the direct light with other light passing near the body, 

 and bent from its direct course : the great difference in the quantity of 



