Article III. 



Experimetits on the Circular Polarization of Light. 

 By H. W. Dove. 



From J. C. Poggendovff's Aimalen der Physik iind Chemie ; Berlin, 

 Second .Series, vol. v. p. 379. 



1 . Circular Polarization of Light by Compressed Glasses. 



▼V HEN two systems of waves, of equal intensity, propagated in the 

 same direction, and polarized perpendicularly to each other, differ in 

 their path by an odd number of quarter-undulations, the particles in 

 the resulting system of waves will describe small circles of a similar 

 velocity around their points of equilibrium ; that is to say, the light will 

 be circularly polarized. Every means of equally satisfying these two 

 conditions, namely, that of the similar intensity of the system of waves 

 polarized perpendicularly to each other, and that of the determinate 

 difference of path, consisting of an uneven number of quarter-undula- 

 tions, will therefore furnish a method of circularly polarizing light. 

 Fresnel and Aiiy have effected this in different ways. The third mode, 

 which I shall hei'e explain, is in practice at least as convenient as those 

 hitherto used, and gives moreover a fuller explanation of the phaenomena 

 of compressed and cooled glasses in polarized light. 



The condition of the equal intensity of the systems polarized per- 

 pendicularly to one another is satisfied by Fresnel by polarizing the 

 incident light in a plane which forms an angle of 45° or 135° with the 

 plane of the total reflexion in a glass pai-allelopiped. The quantities 

 of light polarized in, and also perpendicularly to the plane of reflexion, 

 are then, according to Fresnel's formula of intensity, equal to each other. 

 He obtains the difference of phases of a quarter-undulation by twice- 

 repeated total reflexion, since after a single one under the given circum- 

 stances the periods of vibration of the reflected waves no longer coincide, 

 but exhibit a difference of phases of an |-undulation. 



Tiic mothod which Airy has adopted depends upon another principle. 

 When a thin plate of an uniaxal crystal cut parallel to the axis, and 

 whose axis forms with the plane of polarization of the incident light 

 an angle a, is observed through a riiombohedron of Iceland spar, the 

 principal section of which is inclined toward the plane of primitive 

 of polarization under the angle b, then, if I„, I^ indicate the intensities 



