504 



Mr. P. V. Bevan on Lloyd's 



two pencils of light, one of which has suffered a reflexion ; 

 in the second one pencil of light has suffered one reflexion 

 and the other two reflexions. The Lloyd fringes give us 

 evidence only as to grazing incidence. From the Jamin 

 experiments it appears that when light polarized perpendicu- 

 larly to the plane of incidence is reflected at an angle of 

 incidence between the normal and the principal incidence 

 there is not the loss of the half undulation. It appears also 

 that when light is reflected near the principal incidence, there 

 is a difference of phase introduced in the two components 

 polarized in and perpendicularly to the plane of incidence, 

 giving rise to elliptic polarization in the reflected pencil due 

 to an incident plane-polarized pencil. 



We can easily prove that if there be a loss of half an undu- 

 lation at grazing incidence for the case of all light (the result 

 of Lloyd's experiment),, there must be in the case of light 

 polarized either in or perpendicularly to the plane of 

 incidence a change somewhere between the grazing incidence 

 and normal incidence. It is natural to assume that the light 

 polarized in the plane of incidence suffers no change of this 

 kind, but we should expect at the polarizing angle a change 

 in the character of the reflected beam for light polarized 



Fig. 1. 



perpendicularly lo the plane of incidence. In fig. 1 let P repre- 

 sent the electric vector for incidence nearly grazing, then Q 

 directed in the opposite direction to P as a result of the loss 

 of half an undulation will represent the vector for the reflected 

 light. When we diminish the angle of incidence Ave reach 

 the polarizing angle when Q is zero. If now we diminish 

 the angle still the vector Q must appear on the other side of 



