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tions are now confined to one plane, that passing through 

 the source of light, the reflecting surfoce and the eye. 

 This is termed the plane of polarization, though the vi- 

 brations themselves are at right angles to this plane. 

 While the eye cannot of itself detect this change in the 

 light, there are certain substances which make it appa- 

 rent. Thus the Nicol prism, which consists of two prisms 

 of Iceland spar cemented together by Canada balsam, will 

 allow light to pass through it in one plane only. If now 

 it is true that the vibrations in a reflected beam of light 

 are confined to one plane, when this plane coincides with 

 the plane in the Nicol prism in which light can pass, 

 light should be seen ; but if the prism is turned so that 

 its plane does not coincide with the plane of polarization, 

 darkness should be the result. And this is found to be 

 the case, or would be if the polarization were complete. 

 There are more delicate instruments than the Nicol prism, 

 e. ^., the Savart polariscope which will show polarization 

 if it exists in only four or five per cent, in the light exam- 

 ined. Polarization is not found in direct light, but is 

 found in reflected light. The light of the moon is polar- 

 ized ; the light of the sky is polarized by reflection from 

 the atmosphere, the plane of polarization passing through 

 the sun, the part of the sky examined and the eye. 

 Polarization may also be caused by the light passing 

 through certain crystals, as the Nicol prism, which we 

 have just stated will confine the vibrations to one plane. 

 In this case, the plane of polarization passes through the 

 polarizing crystal and beyond, to the source of the light. 

 In examining the corona with the polariscope, the obser- 

 vation consists in rotating the instrument and if polariza- 

 tion is found, noting the position of the plane of polari- 

 zation. If the plane passes through the sun, the part of 

 the corona examined and the eye, it indicates reflected 



