PHOTOGRAPHY BY POLARIZED LIGHT— McFARLANE 227 



3. Light already polarized will pass through a Pola-screen most 

 freely if the ray is vibrating parallel to the indicator handle. 



4. Light already polarized, which vibrates at right angles to the 

 indicator handle, will not pass through a Pola-screen. 



We can appreciate these peculiar properties, if we arrange two Pola- 

 screens together, as in plate 1, so that the vibration planes are parallel, 

 as indicated when their handles are parallel to each other ; we see that 

 light is transmitted freely. If one Pola-screen is turned so that its 

 handle is at right angles to the other, practically no light is transmitted. 



THE SOURCES OF POLARIZED LIGHT 



It is now evident that we have a tool possessing unique possibilities 

 in controlling Ught. As the photographer is very much concerned 

 in controlling light, this is naturally interesting to him. The value 



Figure 2. — Ray polarized by reflection. A ray of ordinary, unpolarized light is almost completely polarized 

 when specularly reflected at about 32° to any nonmetallic surface, such as glass. This permits subduing 

 oblique reflections from glass and water by a single Pola-screen over the lens. 



of this new device lies in the fact that there are two sources of polarized 

 light in nature: (1) Ordinary, unpolarized light, specularly reflected 

 from any nonmetallic surface at about 32°-37° to the surface, is 

 strongly polarized by the act of reflection, as shown in figure 2. 

 There is some effect at other angles, but none at 0° or 90°. (2) The 

 light from a blue sky, which comes from the region at right angles to 

 the sun's rays, is strongly polarized, as shown in figure 3. 



POLA-SCREEN AT THE LENS ALONE 



Since light rays which are reflected obliquely are polarized, they can 

 be stopped with a Pola-screen, and we can photograph obliquely 

 through glass or water in such a way that we subdue the surface 

 reflections to show detail beyond. The light from the detail beyond 

 the surface is, in general, not polarized, and therefore will be trans- 

 mitted by the Pola-screen at the lens which at the same time will stop 

 the light reflected at the surface. 



OBLIQUE REFLECTION CONTROL 



This property has direct application to photographing store windows 

 where their design is such that an oblique picture is desirable, as 

 shown in plate 2. 



