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' i>' 



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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