This is a property possessed by some bodief, which will, when allowed to move freely, arrange themselves in certain determinate directions, or point, p,s it were, to given poles. Thus, an iron bar acquires polarity by magnetism, and, when suspended from a single point, arranges itself in the direction of the magnetic meridian of the earth. When light is supposed to consist of material particles emitted from the sun, it is necessary to explain certain optical phenomena to assume that the particles are endowed with polarity, which merely signifies that the opposite sides of a particle have different physical properties.