624 



EXPERIMENTS WITH POLARIZED LIGHT 



[Cn. XV 



collections and a few large crystals in museums, no prisms of 

 large openings (5 to 8 cm.) are now to be obtained. 



880. Use of small Nicol prisms. A method will be described 

 for using Nicol prisms of small openings (i to 2 cm.). This 

 method consists in concentrating the light to small diameter in 

 those places where it must pass through the Nicol prisms. 



Let L fig. 350 be the source of light, preferably a small right- 

 angle direct current arc. W, is a water-cell. It is imperative to 

 use a water-cell, otherwise the polarizing Nicol will be ruined. 

 The polarizing Nicol NI, is placed at the image of the 

 crater. Light spreading out from the farther side of this 

 prism is polarized. This Nicol is treated exactly 

 as if it were an original source of polarized light. 

 The second condenser Ca, which must be free 



FIG. 351. LIGHT POLARIZED BY A PILE OF GLASS PLATES. 

 The light source. 

 Plano-convex condenser lens. 

 Condenser of long focus and free from strain. 

 The specimen. 

 Magic lantern objective. 



N 2 Nicol prism with I to 2 cm. opening (analyzer). 



from strain, renders the light parallel or slightly convergent. The 

 specimen is at S. The objective O, is placed at such a distance 

 from the specimen that the image of the latter will be in focus on 

 the screen. The analyzing Nicol N 2 , is placed at the right in front 

 of the objective at the point where the rays cross, i. e., in the image 

 of NI, cast by the lenses C 2 and 0. 



When either the polarizing or analyzing Nicol NI or N 2 , is rotated, 

 two positions, 180 apart, will be found in which the screen is dark. 

 If when these positions are found, a piece of mica, for example, is 

 put in the field, it will change the plane of polarization and will give 

 on the screen the most beautiful colors. 



