156 SCIENTIFIC RECREATIONS. 



they occur only in one plane the light is said to be " polarized." Polarization 

 means possessing poles (like a magnet) ; the polarized rays have " sides," as 

 Newton said, or, as explained by Dr. Whewell, " opposite properties in 

 opposite directions, so exactly equal as to be capable of accurately neu- 

 tralizing each other." There are some crystals which possess the property 

 of " double refraction," and thus a ray of common light passing through such 

 a crystal is divided into two polarized rays, taking different directions. One 

 is refracted according to the usual laws of refraction; the other is not, and 

 the planes of polarization are at right angles. It is difficult within the limits 

 of this chapter to explain the whole theory of Polarization. In order to 

 account for certain phenomena in optics, philosophers have assumed that 

 rays possess polarity ; and polarized light is light which has had the 

 property of Polarization conferred upon it by reflection, refraction, or 

 absorption. Common light has been compared to a round ruler, and polarized 

 light to a flat ribbon. Huygens found out, when engaged upon the inves- 

 tigation of double refraction, that the rays of light, divided by passing through 

 a crystal (a rhomb) of Iceland spar, possessed certain qualities. When he 

 passed them through a second rhomb, he found that the brightness, relatively, 

 of the rays depended upon the position of the second prism, and in some 

 positions one ray disappeared entirely. The light had been reduced to 

 vibrations in one plane. In 1808, Malus, happening to direct a double 

 refracting prism to the windows then reflecting the sunset, found that as he 

 turned the prism round, the ordinary image of the window nearly disappeared 

 in two opposite positions ; and in two other positions, at right angles, the 

 " extraordinary " image nearly vanished. So he found that polarization was 

 produced by reflection as well as by transmission. The differences between 

 common and polarized light have been summed up by Mr. Goddard as 

 follows : 



COMMON LIGHT POLARIZED LIGHT 



" Is capable of reflection at oblique angles " Is capable of reflection at oblique angles 



of incidence in every position of the only in certain positions of the reflector. 



reflector. 



" Will pass through a bundle of plates of " Will only pass through such glasses when 



glass in any position in which they may they are in certain positions. 



be placed. 



"Will pass through a plate of tourmaline, "Will only pass in certain positions, and in 



cut parallel to the axis of the crystal, in others will not pass at all." 



every position of the plate." 



The bundle of glass plates or the tourmaline plate is thus the test for 

 polarized light, and is termed an analyzer. 



The arrangement called a " Nichol's prism," made by cutting a prism 

 of Iceland spar and uniting the halves with a cement, so that only one polar- 

 ized ray can pass through it, is termed a Polarizer. It only permits one of 

 the two rays produced by " double refraction " to pass, and the ray (as said 

 above) will contain none but transverse vibrations. Polarized light will 



