THE MICROSCOPE. 



The name POLES is given in physics in general to the sides or 

 ends of any body which enjoy or have acquired any contrary pro- 

 perties. Thus, the opposite ends or sides of a magnet, have con- 

 trary properties, inasmuch as each attracts what the other repels. 

 The opposite ends of an electric or galvanic arrangement are, for 

 like reasons, denominated poles. 



70. Following the common rule of analogy in nomenclature, a 

 ray of light which has been submitted to reflection or transmission 

 under the special conditions referred to, has been called polarised 

 light ; inasmuch as it is found that the sides of the ray which 

 lie at right angles to each other, possess contrary physical pro- 

 perties, while those of a ray of common or unpolarised light possess 

 the same physical properties. 



To illustrate the relative physical condition of common light 

 and polarised light, we may compare a ray of common light to a 

 round rod or wire of uniform polish and uniformly white, while 

 a ray of polarised light may be compared to a similar wire, two 

 of whose opposite sides are rough and black, while the other 

 opposite sides at right angles to these are polished and white. 

 Thus, if A B c D, fig. 33, be a section of the former, the entire 

 circumference A B c D is white and polished, and if A' B' c' D' 



be a section of the latter, A' B' and c' D' will be white and polished, 

 while B' c' and D' A' will be black and rough. 



A group of physical properties, very numerous and complicated, 

 characterise the polarised state of light, the discussion and exposi- 

 tion of which, constitute the subject of an extensive and important 

 section of optics. It would be obviously impossible here to convey 

 to the reader any general idea of these ; nevertheless, as an illus- 

 tration of them, one of the most frequent occurrence may be 

 mentioned. If a ray of common light fall upon a smooth and 

 polished surface, it is always reflected according to the well- 

 known laws of reflection, no matter what side of it may be pre- 

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