REPORT ON PHYSICAL OPTICS. 409 



by an helicoidal arrangement of the molecules of the vibrating 

 medium, which will have different properties according as the 

 helices are right-handed or left-handed. But this hypothesis 

 can hardly be supposed to apply to the case of fluids, in which 

 the property of circular polarization is independent of direction j 

 and we are driven to confess that, with respect to these import- 

 ant laws, physical theory is as yet wholly at fault. The singular 

 relation between the interval of retardation and the length of the 

 wave seems to afford the only clue to the xmravelling of this 

 difficulty. 



The phenomena of depolarization and of colour, impressed 

 by double- refracting substances upon the transmitted light, are, 

 we have seen, the necessary results of the interference of the 

 two pencils into which the light is divided within them. These 

 properties, then, enable us to discover the existence, and to 

 trace the laws of double-refraction, even in substances in which 

 the separation of the two pencils is too minute to be directly 

 observed. By such means the important discovery has been 

 made, that a double-refracting structure may be communicated 

 to bodies which do not naturally possess it, by mechanical com- 

 pression and dilatation. Sir David Brewster observed that 

 when pressure was applied to the opposite faces of a parallelo- 

 piped of glass, it developed a tint in polarized light, like a 

 plate of a double-refracting crystal ; and the tint descended in 

 the scale as the pressure was augmented. Singly-refracting 

 crystals, such as muriate of soda and fluor spar, acquired the 

 properties of double refraction by the same means *. All this 

 is in perfect accordance with the wave-theory. Owing to the 

 connexion of the vibrating medium with the solid in which it is 

 contained, its elasticity is rendered unequal in different direc- 

 tions by the effect of compi'ession, the maximum and minimum 

 corresponding to the directions of greatest and least pressure. 

 Accordingly, the vibrations of the ray on entering the plate are 

 resolved into two in these rectangular directions, and these are 

 propagated with unequal velocities ; the colour developed is 

 determined by the interval of retardation. These results of 

 theory were experimentally confirmed by Fresnel, by the me- 

 thod of interferences ; and it was found that the velocity with 

 which a ray traversed the glass was greater or less, according 

 as it was polarized pai'allel or perpendicular to the axis of com- 

 pression. The bifurcation of the ray at oblique incidences is a 

 necessary consequence of this difference of velocities ; but this 

 was also shown by Fresnel by direct experiment. A series of 



• Phil. Trans. 1 8 1 5 and 1 8 1 6. 



