THE ANALYSIS OF LIGHT. 343 



Tho transmitted ray, having all its vibrations in ono 

 direction, readily passes through a second plate of 

 tourmaline, the structural arrangement of which ia 

 symmetrical with that of the first; but if this ar- 

 f| I I " rangement be altered by turning the plate partially 



__J_JJ round, the vibrations are intercepted. In the same 



manner a sheet of paper, c c?, Fig. 279, may be slipped 

 through a grating, a 6, its plane coinciding with the length of the bars ; but 

 caji no longer go through when it is turned, as at e / a quarter round. 



Light is polarized by reflection from many 

 izcd"by refloc- diflFeront substances, such as glass, water, air, 



Won from other -, ,\ c ^ j^ c , i 



substances eoonj, mother-oi-pearl, suriaces oi crystals, 

 augasb ^^^^ ^^^^ provided that the light falls at a 

 certain angle peculiar to each surface. This angle is 

 called the polarizing angle.* 



„_ ^ Since the discovery of polarized light, its principles have 



What are some i.,i-i ..« .. 



ef the practical been applied to the determmation of many practical rcsulta 



app!ication<! of ^^^ jj. jj^g been found that aU reflected light, come from 

 polarized light? ' ^ 



whence it may, acquires certain properties which enable us 



to distinguish it from direct light ; and the astronomer, in this way, is en- 

 abled to determine with infallible precision whether the light he is gazing on 

 (and which may have required hundreds of years to pass from its source to 

 the eye), is inherent in the luminous body itself, or is derived from some other 

 source by reflection. It has been also ascertained by Arago that light pro- 

 ceeding from incandescent bodies, as red-hot iron, glass, and liquids, under a 

 certain angle, is polarized light ; but that light proceeding, under the same 

 circumstances, from an inflamed gaseous substance, such as is used in street 

 illumination, is always in a natural state, or unpolarized. Applying these 

 principles to the sun, ho discovered that the light-giving substance of this 

 luminary was of the nature of a gas, and not a red-hot solid or liquid body. 



In a similar manner the chemist is able to determine, by the manner in 

 which light is reflected or polarized by a crystallized body, whether it has 

 been adulterated by the addition of foreign substances. 



What three "^^3. Solar light, iu addition to the lumin- 

 fnduded* "i^ ous principle which produces the phenomena of 

 •oiar light? color and is the cause of vision, contains two 

 other principles, viz., heat and actinism, or the chem- 

 ical principle. These principles are invisible to the eye, 

 and have only been discovered by their effects on other 

 bodies. 



• The phenomena of polarized light are so abstruse, and depend to so great an extent 

 on experimental illustration for their proper comprehension, that on extended descrip- 

 tion of them in an elementary work is impossible. 



