30 FOODS AND FOOD ADULTERANTS. 



which is most refracted is called the ordinary and the other the extraor- 

 dinary ray. The refractive index of tbe film of balsam being inter- 

 mediate between those of the rays, permits the total reflection of the 

 ordinary ray, which, passing to the blackened sides of the prism, is ab- 

 sorbed. The extraordinary ray passes the film of balsam without de- 

 viation and emerges from the prism in a direction parallel with the 

 incident ray, having, however, only^half of its luminous intensity. 



Two such prisms, properly mounted, furnish the essential parts of a 

 polarizing apparatus. They are called the " polarizer" and the "ana- 

 lyzer," respectively. 



If now the plane of vibration in each prism be regarded as coincident 

 with its principal section, the following phenomena are observed: If 

 the prisms are so placed that the principal sections lie in the prolonga- 

 tion of the same plane, then the extraordinary polarized ray from the 

 polarizer passes into the analyzer, which practically may be regarded 

 in this position as a continuation of the same prism. It happens, there- 

 fore, that the extraordinary polarized ray passes through the analyzer 

 exactly as it did through the polarizer, and is not reflected by the film 

 of balsam, but emerges from the analyzer in seemingly the same con- 

 dition as from the polarizer. If now the analyzer be rotated 180, 

 bringing the principal section again in the same plane, the same phe- 

 nomenon is observed. But if the rotation be in either direction only 90, 

 then the polarized ray from the first prism, incident on the second, de- 

 ports itself exactly as the ordinary ray, and on meeting the film of bal- 

 sam is totally reflected. The field of vision, therefore, is perfectly dark. 



In all other inclinations of the planes of the principal sections of the 

 two prisms the ray incident in the analyzer is separated into two, an ordi- 

 nary and extraordinary, varying in luminous intensity in proportion to 

 the square of the cosine of the angle of the two planes. 



Thus by gradually turning the analyzer, the field of vision passes 

 slowly from maximum luminosity to complete obscurity. 



The expression " crossed Nicols " refers to the latter condition of the 

 field of vision. 



Selenite plate. In the practical application of polarized light to the ex- 

 amination of facts, an important use is made of a seleuite plate (crystal- 

 lized sulphate of calcium). A disk of selenite, interposed between the 

 polarizer and analyzer imparts a coloration to the field of vision which 

 varies with the relative position of the principal sections of the two 

 prisms. 



This phenomenon depends on the fact that a plane polarized ray of 

 light can be decomposed, in passing a section of a bi-refracting crystal 

 like selenite or mica, into two rays, polarized at right angles and dif- 

 fering in phase. 



This fact is illustrated by passing a polarized ray (from a Nicol prism) 

 through a very thin crystallized plate of mica or gypsum (selenite) 

 obtained by cleavage. By the double refraction of the thin plate the po- 



