THE WAVE THEORY OF LIGHT. 333 



Plane-polarised light is light with the vibrations 

 all in a single plane, perpendicular to the plane 

 through the ray which is technically called the 

 "plane of polarisation." Circularly polarised light 

 consists of undulations of luminiferous ether having 

 a circular motion. Elliptically polarised light is 

 something between the two, not in a straight line, 

 and not in a circular line ; the course of vibration 

 is an ellipse. Polarised light is light that performs 

 its motions continually in one mode or direction. 

 If in a straight line it is plane-polarised ; if in 

 a circular direction it is circularly polarised 

 light ; when elliptical it is elliptically polarised 

 light. 



With Iceland spar, one unpolarised ray of light 

 divides on entering it into two rays of polarised 

 light, by reason of its power of double refraction, 

 and the vibrations are perpendicular to one another 

 in the two emerging rays. Light is always polarised 

 when it is reflected from a plate of unsilvered glass, 

 or from water, at a certain definite angle of fifty- 

 six degrees for glass, fifty-two degrees for water, 

 the angle being reckoned in each case from a 



