226 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



plane is given by the engraved line on the indicator handle. In 

 figure 1 ordinary Ught is coming from the left. It is vibrating in all 

 directions. It then passes through the polarizer at the left, the 

 vibration plane of which happens to be vertical. The ray is polarized, 

 vibrating in a vertical plane only. If this polarizer had been placed 

 with its vibration plane horizontal, then the rays would vibrate in a 



horizontal plane. 



If a second polarizer is placed in the path of light which has left 

 the first polarizer, another interesting property is brought out. If the 



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Figure 1 -Light rays are polarized by the Pola-screen at the left, the vibration plane of which is shown 

 vertically. The amount of light let through by the second Pola-screen is cut down as the Pola-scrcon 'S 

 turned. When the vibration plane of the second Pola-screen is at right angles to the first, practically no 

 oblique light gets through. 



vibration plane of the second polarizer is parallel to that of the first, 

 the ray is transmitted freely, still vibrating in the same plane, as 

 shown in the upper dramng of figure 1. The polarizers are then said 

 to be parallel. But if the second polarizer is rotated, the mtensity of 

 the light is gradually cut down, and when the vibration planes of the 

 two polarizers are at right angles, as in the lowest drawing of figure 1, 

 practicaUy no fight is allowed through. The polarizers are then said 



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These properties are summarized in terms of a Pola-screen thus: 



1. Light transmitted by a Pola-screen vibrates in a direction 

 parallel to the indicator handle. 



2. This dhection of vibration rotates as the Pola-screen is rotated. 



