POLARIZATION OF LIGHT 



4731 



POLECAT 



through most transparent substances without 

 obstruction. But a few substances, especially 

 tourmaline (which see) and Iceland spar, ob- 

 struct some of the vibrations. 



Allow a ray of light from the sun or a lamp 

 to pass vertically through a crystal of tourma- 

 line in a dark room and fall upon a screen. 



b a a 



FIG. 1 FIG. 2 



In the first figure light Is passed through tour- 

 maline crystals a b. In the second figure light is 

 cut off by crossed tourmaline crystals. 



There is no apparent change in the light. Place 

 another crystal of tourmaline between the first 

 and the screen so that the surfaces of the two 

 crystals are parallel. Revolve the second crys- 

 tal about an imaginary horizontal axis. At two 

 points in the revolution, 180 apart, some of 

 the vibrations will be obstructed, and this 

 change will be shown on the screen. Fig. 2 is 

 an illustration of what occurs. The tourmaline 

 allows the transverse vibrations of light waves 

 to pass through it in one direction only. This 

 limitation of light waves to a single direction is 

 called polarization of light. 



Iceland spar (which see) has the property of 

 polarizing light by double refraction ; that is, it 



Kic;. 3 



separates the rays of light into the ordinary 

 and the polarized, or extraordinary, rays. Ob- 

 jects seen through a crystal of Iceland spar ap- 

 pear double, as shown, in Fig. 3. Light may 

 also be polarised by reflection from such sur- 

 faces as glass, water and other nonmetallic 

 substances. The polarization may be recog- 

 nized by the rainbow tints appearing on the 

 reflecting surface. Soap bubbles produce a 

 :Trrt. The effect is not produced un- 

 less the light strikes the reflecting surface at 



the proper angle, which is known as the angle 

 of polarization. The angle varies for different 

 substances. 



Some transparent substances turn polarized 

 light to the right and others turn it to the left. 

 This fact is sometimes used for detecting the 

 adulteration of sugar. Cane sugar turns the 

 light to the right and fruit sugar turns it to 

 the left, and by subjecting a sample of sugar to 

 the test of polarized light the proportion of 

 fniit sugar is easily detected. See POLARISCOPE. 



POLE, either end of the axis about which 

 the earth turns. The northern extremity, which 

 is ninety degrees north of the equator, is called 

 the North Pole ; the South Pole occupies a cor- 

 responding position at the southern end. As- 

 tronomers use the term celestial pole to desig- 

 nate that point in the heavens at which the 

 earth's axis points and about which the stars 

 appear to revolve. The star nearest this point 

 in the northern heavens, called the Poll 

 is used by sailors in keeping their direction on 

 the sea. In a broad sense, the term pole may 

 be used to designate a point on any sphere 

 situated as the earth's poles are to the equator. 

 Thus, the poles of the horizon are the zenitli 

 and the nadir. In physics the term is used to 

 designate the points of a body at which its at- 

 tractive or repulsive force is concentrated. The 

 magnetic poles of the earth are points on the 

 surface where the magnetic needle is vertical, 

 or upright (see MAGNET AND MAGNETISM). 



POLE 'CAT, an animal belonging to the 

 weasel family, closely related to the skunk of 

 North America. It was once widely distributed 

 throughout Europe, but is now bring extermi- 

 nated in many places because of its destructive 

 habit of feeding on domestic fowls of various 

 sorts, as well as on rats, mice, eggs and wild 

 birds. In common with the skunk, it secretes, 

 and can discharge at will, a liquid of a most 



THE POLECAT 



disagreeable odor. The common variety is 

 about seventeen inches long, with a tail six 

 inches in length. Its coarse brown fur, turning 

 black below, is used for artists' brushes. The 

 an skunk is sometimes incorrectly called 

 polecat. 



