142 PHYSICAL PROPERTIES OF POLARIZED LIGHT. 



a transparent medium, as glass, in whatever position it 

 may be placed relative to the source of light. 



A polarised ray of light is not transmitted in all the 

 positions of the permeable medium. 



Supposing a plate of glass is presented at the angle 

 56 to a polarised ray, and the plane of incidence or 

 reflexion is at right angles to the plane of polarisation 

 of the ray, no light is reflected. If we turn the plate of 

 glass round through 90, when the plane of reflexion is 

 parallel to that of polarisation the light is reflected. If 

 we turn the plate round another 90, so that the plane 

 of reflexion and of polarisation are parallel to each other, 

 again no light is reflected ; and if we turn it through 

 another 90 the reflection of the ray again takes place. 



Precisely the same result takes place when, instead of 

 being reflected, the polarised ray is transmitted. 



Some substances have peculiar polarizing powers : 

 the tourmaline is a familiar example. If a slice of 

 tourmaline is taken, and we look at a common pencil 

 of light through it, we see it in whatever position we 

 may place the transparent medium. If, however, we 

 look at a pencil of polarised light, and turn the crystal 

 round, it will be found that in two positions the light is 

 stopped, and that in two other positions it passes freely 

 through it to the eye. 



By way of endeavouring to conceive something of 

 what may be the conditions which determine this very 

 mysterious state, let us suppose each ray of light to 

 vibrate in two planes at right angles to each other : one 

 wave being vertical and the other horizontal. We have 

 many examples of this compound motion. The mast of 

 a ship, by the force with which she is urged through 

 the water, describes a vertical wave, while by the roll of 

 the billows across which she sails, a lateral undulation 

 is produced at the same time. We may sometimes 

 observe the same thing when a field of corn is agitated 

 by a shifting wind on a gusty day. 



