52 THE POPULAR SCIENCE MONTHLY. 



tion by the interposition of this plate of quartz between the reflecting 

 surface and the Nicol. The changes in the light are no longer mere 

 alterations of brightness, but exhibit a succession of colors resem- 

 bling in their main features those of the rainbow or spectrum. 



The peculiar condition to which light must be brought in order 

 that these phenomena may be produced is called polarization ; and, al- 

 though an explanation of its nature must be reserved until later, I beg 

 you to notice that it is effected in this instance by reflection from a 

 plate of glass. A similar effect is produced if light be reflected from 

 many other substances, such as the leaves of trees, particularly ivy, 

 mahogany furniture, windows, shutters, and often roofs of houses, oil- 

 paintings, etc., and last, but not least, the surface of water. In each 

 of these cases the alternations of light and darkness are most strongly 

 marked, and the colors (if a quartz plate be used) are most vivid, or, 

 in technical language, the polarization is most complete, when the light 

 is reflected from each substance at a particular angle. In proportion 

 as the inclination of the light deviates from this angle the colors be- 

 come fainter, until, when it deviates very greatly, all trace of polariza- 

 tion at last disappears. Without occupying the time necessary to 

 shift our apparatus so as to exhibit this with the glass plate, we may 

 alter the reflecting surface from glass to water, and, by projecting on 

 the screen the beautiful phenomena of liquid waves, make visible the 

 different degrees of polarization produced at the variously-inclined 

 portions of the surfaces of those waves. A tea-tray will serve as well 

 as any thing else to form our little sea, and a periodic tap at one corner 

 will cause ripple enough for our present purpose. The waves now ap- 

 pear bright on the screen, and, although brighter in some parts than 

 in others, they are nowhere entirely dark. But on turning round the 

 Nicol the contrast of light and darkness becomes much stronger than 

 before. Here and there the light is absolutely extinguished ; in these 

 parts the polarization is complete, in others incomplete in various de- 

 grees. And if the quartz plate be again introduced we have the beau- 

 tiful phenomena of iris-colored rings playing over the surface of our 

 miniature sea. 



Now, that which you see here produced by our lamp and tea-tray, 

 you may see any day under the bright sky of this southern coast. 

 By using an apparatus such as we have here, or a simpler one which 

 I will immediately describe, you may bring out for yourselves these 

 phenomena of color, and thereby detect the profusion of polarization 

 which Nature sheds around us. But, before describing it, there is one 

 peculiar feature of all these experiments which must be noticed 

 namely, that the same results would be produced if we changed the 

 positions of the lamp and the screen. The light which is now polar- 

 ized by the glass or the water, and examined by the Nicol, might 

 equally well be polarized by the Nicol and examined by the glass or 

 the water. And, therefore, if we find that any contrivance will serve 



