Polarized Light through Douhle-refracting Crystals. 137 



piece through which the ray, having passed, is lastly analyzed by 

 the second Nicol's prism. 



The loss of light from the number of the above media is not so 

 great as might be supposed, still an intense source of light is desirable 

 for satisfactory results. A good artificial light placed close to the 

 mirror will be found the best. In diffused daylight rays are apt 

 to enter the object-glass by reflexion from the brasswork without 

 first passing through the polarizer, by which the beauty of the 

 spectrum is impaired. 



To understand the bearing of the experiments, it is necessary 

 to keep in view the different effects of a doubly-refracting film upon 

 polarized light, according to the position of its axes, with respect to 

 the planes of polarization. 



Suppose we take a film of selenite, such as those commonly sold 

 as an adjunct to the polarizing microscope, giving, as its two colours, 

 a pinkish red and its complementary green. Such a film will, if 

 examined between two Nicol's prisms, act on the light according to 

 the following laws : — 



1st. When a neutral axis of the film is in the plane of primitive 

 polarization, the film will exercise no influence on the light ; if 

 therefore the prisms are set with their axes perpendicular the field 

 will remain dark, if the prisms have their axes parallel the field will 

 contain only white light. 



2nd. If the prisms are placed with their axes perpendicular, 

 and the film is made to rotate, there will be four points of darkness 

 at each quarter of a revolution, viz. when an axis of the film is in 

 the plane of polarization, and between these four points, the same 

 colour (say green) wiU occur. 



3rd. If the prisms are set with their axes parallel, and the 

 selenite is rotated, the field will be white at the four points where 

 it was previously dark, and of the complementary colour (red) be- 

 tween each of these four points. 



4th. If the selenite is fixed with its neutral axis inclined 45° 

 to the plane of primitive polarization, and the analyzer made to 

 rotate, the field will be alternately red and green in the four 

 quadrants. 



5th. The colours are always of maximum brightness when the 

 axes of the prisms are perjjendicular or parallel, and the axes of the 

 selenite inclined 45° to the plane of polarization. 



Suppose, now, we repeat the above experiments, using the 

 polarizing spectrum microscope above described, and let us call the 

 point in the revolution of the selenite at which either of its axes is 

 in the plane of primitive polarization, the zero point, from which 

 the number of degrees through which it is turned are measured. 



Let the prisms be set with their axes perpendicular to one 

 another, and the selenite rotated on the stage. The spectrum will 



L 2 



