OPTICAL PRINCIPLES. 



these through a tourmaline or a NicoPs prism, as in 

 the experiment with the two tourmalines, the images 

 will become alternately visible and invisible, just as 

 was then the case with the entire mass of light. 



The light which has undergone this singular change 

 is said to be polarized, because the rays appear to 

 have acquired poles or sides. In the above experi- 

 ments the lower prism or tourmaline is called the 

 polarizer, because it polarizes the light, and the upper 

 is called the analyzer, because it analyzes or tests the 

 light altered by the former. 



An idea of the cause of this change may be ob- 

 tained by reference to the undulatory theory of light. 

 Ordinary light consists of waves or undulations taking 

 place in planes at right angles to each other, or in all 

 planes; while in polarized light the undulations are 

 all in one plane or in parallel planes. This may per- 

 haps be understood by considering that books in a 

 book-case are situated in parallel planes, the shelves 

 being in planes at right angles to the former. And 

 by imagining in polarizing substances the existence of 

 some structure acting like a grating, a notion can be 

 obtained how the rays in the different planes may be 

 transmitted or intercepted. If the grating be so 

 placed that the bars (representing the planes of polari- 

 zation) are perpendicular, the books can pass between 

 them ; while if the grating be turned round a quarter 

 of a circle, they will become transverse, and the books 

 cannot pass, while the shelves could do so. Carrying 

 on this analogy, the tourmaline or NicoPs prism po- 

 larizes the light by transmitting only those rays whose 

 undulations are in planes parallel to the bars ; while 

 the analyzer allows these undulations to pass through 

 it when the direction of the planes coincides with that 

 of the bars, but interrupts them when their direction 

 is at right angles to the bars. And it is evident that 

 the planes of polarization of the ordinary and extra- 

 ordinary rays are opposite, from the opposite action 

 of the analyzer upon them. 



