144 REPORT ON PHYSICAL OPTICS. 



light and in any position of the analyzing plate; and he has 

 found the most striking agreement between the results of calcu- 

 lation and those of observation. 



We yet want a mechanical theory which will account for the 

 peculiar form of the wave-surface just alluded to. Fresnel seems 

 to have thought that the difference of the velocities of the two 

 rays in the direction of the axis might be physically explained 

 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 ; 

 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 betweent he interval of retardation and the length of 

 the wave seems to afford the only clue to the unravelling 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 compression and 

 dilatation. Sir David Brewster observed that when pressure was 

 applied to the opposite faces of a parallelepiped of glass, it deve- 

 loped 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 differ- 

 ent directions by the effect of compression, the maximum and 

 minimum corresponding to the directions of greatest and least 



* Phil Trrms. 1815 and 1816. 



