POLARIZED LIGHT. 97 



in a second of time. White light is produced 

 when the undulations are 44,440 in an inch, and 

 their number in a second of time amounts to 541 

 millions of millions. These vibrations are com- 

 municated to the retina and optic nerve, and from 

 thence to the brain. The rapidity with which 

 these undulations are communicated from their 

 source to the eye may be imagined when it is 

 stated that the light from the sun (a distance of 

 about 90 millions of miles) reaches us in 8 minutes 

 and 13 seconds; a railway train travelling at the 

 speed of 60 miles an hour would require 180 years 

 to accomplish the same distance. The light from 

 remotest nebula (according to Sir W. Herschel) 

 would, however, require 2,000,000 years to reach 

 the earth. 



A ray of common light is supposed to have at 

 least two sets of vibrations ; viz., one vertical (or 

 up and down), and the other horizontal (or from 

 side to side). 



These vibrations are capable of being separated 

 either by reflection or by passing the ray through 

 certain transparent substances. The light is then 

 said to be polarized. The name is not, perhaps, 

 the best that could have been chosen, but as it 

 has been in use for many years, any alteration 

 would be attended with inconvenience. 



The terms poles and polarity are usually em- 

 ployed to describe the contrary properties pos- 

 sessed by the opposite ends of bodies. Thus, we 

 have the north and south poles of a magnet, one 

 of which attracts what the other repels ; and when 

 it was found that the sides of a beam of light, 

 when reflected or transmitted under certain con- 

 ditions, possessed opposite properties, the ray 

 was said to be polarized from a fancied resem- 

 blance to the poles of a magnet or galvanic battery, 



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