LIGHT. 127 



POLARIZATION OF LIGHT. 



Plane-polarized light may be obtained in great 

 quantity by using for a reflector in the porte lumiere 

 a plate of glass blackened upon its back surface. 

 Choose a piece of good window-glass, without bubbles 

 or striae, and paint it upon one side with lamp-black 

 mixe4 in Japan varnish. It will be best to lay on two 

 or three coats in order to completely cover the surface. 

 Hold it between the eye and the sun, and all the 

 uncovered and thin places can be seen ; they should 

 receive another coat of paint. This painted glass 

 should be of the same size as the plane mirror in the 

 porte lumiere, upon which it may now be placed and 

 fastened by tying about them both a thread or stretching 

 a ring of elastic cord over them. If the beam of light 

 which is now reflected from the unpainted surface of 

 this glass is sent through a double-convex lens and 

 then received upon the screen, it will be seen to be 



Tig. 95. 



much less intense than the beam from the silvered 

 mirror, but some of the most attractive experiments in 

 the whole domain of physics are possible with this 

 light. 



A Nicol's prism n will be necessary, and the larger 

 it is the better, but very gbod eflects may be obtained 



