PLANE POLARIZATION. 223 



fleeted at the surface of water at an angle of 37, or 

 from the surface of glass at an inclination of 35 25 ', is 

 as completely polarized as the two rays, ordinary and 

 extraordinary, proceeding from a crystal of Iceland spar. 



The reflexion of light long ago occupied observers in 

 the age of Plato and of Euclid : since that epoch it had 

 been the object of thousands of experiments, of hundreds 

 of theoretical speculations ; the law according to which it 

 proceeds serves as the basis of a great number of instru- 

 ments, ancient and modern. Among the multitude of 

 enlightened minds, of men of genius, of skilful artists, 

 who, during more than 2300 years, have been occupied 

 with this phenomenon, no one ever aimed at any other 

 object than the means of making the rays divide, or of 

 causing them to diverge or converge ; no one ever imag- 

 ined that reflected light ought not to possess all the same 

 properties as the incident light, or that a change of path 

 would be the cause of a change of nature. Generations 

 of observers thus succeeded each other during several 

 thousands of years, every day touching closely on the 

 most beautiful discoveries without actually making them. 



Malus, as I have already explained, gave a means of 

 polarizing light different from that which Huyghens had 

 formerly announced. But the polarizations produced by 

 the two methods were identically the same. The re- 

 flected rays and those which proceed out of an Iceland 

 crystal possess exactly the same properties. Since that 

 time a member of this Academy (Arago) has discovered 

 a kind of polarization* entirely distinct, and which mani- 



* It may be necessary for some readers to explain that, in this some- 

 what paradoxical mode of speaking, the author is referring to his own 

 discovery of the polarized tints ; and his meaning is simply that if, in 

 polarized light, there be placed a thin film, e. g., of selenite or mica, 

 and it be viewed through a doubly refracting crystal as an analyzer, 



