DAIRY PRODUCTS. 31 



larized ray is separated into two, ordinary and extraordinary. The ex- 

 traordinary, having to pass over a greater distance, joints the ordinary 

 ray, after emergence, with a phase slightly different, the degree of dif- 

 ference depending on the nature of the lamina, the inclination of the in- 

 cident ray, &c., but in every case this difference of phase can be easily 

 calculated, and the resultant beam of light is said to be elliptically po- 

 larized. Each of the components of this ray enters the analyzer and is 

 again resolved. One of its elements is suppressed in the Nicol and the 

 other, consisting of vibrations in the principal plane, passes through. 

 The result is two sets of vibrations in the same plane slightly different 

 in phase, which are, therefore, in a condition to interfere and produce 

 color. If the source of light be monochromatic, when the analyzer is 

 rotated, only certain variations in luminous intensity will be observed ; 

 but if, on the other hand, white light be employed these variations in 

 phase will give rise to a display of colors, In order that the field of 

 vision be of a uniform tint it is necessary that the lamina of crystal be 

 of uniform thickness. For ordinary use the selenite plate b ground to 

 a thickness which will give green and red tints. 



For crossed Mcols the colors of the selenite plate appear brightest 

 when it is so placed that the plane of vibration in the crystal forms an 

 angle of 45 with the plane of vibrations of the polarized incident ray. 

 If the seleuite plate is rotated in its own plane, the color appears in the 

 four quadrants at its maximum and disappears at intervals of 90. 



If the planes of the two Nicols are parallel, the same order of phe- 

 nomena appear as before, except that the positions of maximum and 

 minimum are reversed. 



If the analyzer be rotated and the selenite plate and polarizer re- 

 main stationary there is no effect produced, when the principal section 

 of the selenite is parallel or perpendicular to the polarizing plane of 

 the under Nicol. But if this plane is inclined less than 45 to that of 

 the polarizer, then the selenite plate in a complete revolution of the 

 analyzer will appear four times brightly colored and four times color- 

 less. In adjoining quadrants the colors will be complementary. When 

 the Nicols are so placed as to produce the maximum intensity of color, 

 if small bi-refracting crystals be introduced at random into the field of 

 vision, they will, in general, have the same effect on the plane polar- 

 ized ray as the selenite plate. Since the axes of these crystals may 

 have any accidental position with reference to the planes of the Nicols, 

 it follows that the field of vision, which before appeared of a uniform 

 tint, will now become variegated, the color disappearing in some cases 

 ind becoming more intense in others. 



When a bi-refracting crystal is cut into laminae normal to its axis, 

 Df appropriate thickness, it gives some peculiar phenomena when ex- 

 amined with polarized light. When the analyzer is perpendicular to 

 the polarizer, there is seen in the ordinary image a black cross, the ex- 

 istence -of which can be explained by the mathematical theory of polar- 



