=414. Polarization of Light.=--It is found that two crystals of tourmaline behave toward light just as the two gratings behave with respect to the transverse waves of the rope. Thus, if a small opening in a screen is covered with a _tourmaline_ crystal, light comes through but slightly diminished in intensity. If a second crystal is placed over the first one so that the two axes are in the same direction as in Fig. 412_P_, light is as freely transmitted through the second crystal as through the first, but if the crystals are crossed (Fig. 412_S_) no light passes the second crystal. This experiment shows that the light which has passed through one tourmaline crystal will pass through another only when the latter is held in a certain position, hence it is believed that a tourmaline crystal is capable of transmitting light that is vibrating in one particular plane. The direct conclusion from this is that _light waves_ are _transverse rather than longitudinal_. The experiment just described illustrates what is called _polarization of light_. The beam that after passing through _a_ (Fig. 412) is unable to pass through _b_, if the two axes are crossed, is called a _polarized beam_. The conclusion that light waves are transverse is therefore based upon the phenomenon of the polarization of light. This was first discovered by Huygens in 1690. Important Topics 1. Interference of light: evidence, reasoning involved, illustration. 2. Polarization of light: evidence, reasoning involved. 3. Nature of light, differences between sound and light. Exercises 1. Make a list of the differences between sound and light and state briefly the evidence upon which the knowledge of these differences is based. 2. Why will a thickness of film that will produce interference of red light be different from that producing interference for green or blue? 3. Using the formula _n_ = _v_/_l_ compute the vibration rate for violet light if its wave length is considered as 0.00004 cm. 4. Explain how the fact of polarization affects the wave theory of light. 5. Show how it is possible by comparing the spectrum of the sun with that of a star to tell whether the star is approaching or receding from the earth. Review Outline: Light Light; speed, source, medium. Straight Line Motion; shadow, umbra, penumbra, eclipse, image. Photometry; Law of intensity, candle power, foot-candle. Mirrors; Law of reflection; image--real, virtual; plane, curved, parabolic, mirrors. Refraction; cause and effects; plate, prism, lens; total reflection. Lenses; six forms, principal focus, center, lens equation, 1/_F_ = 1/_D_{o}_ + 1/_D_{i}_. Optical instruments; eye, defects and correction, camera, microscope, etc. Spectra; 3 kinds, dispersion, production of color effects, spectroscope, uses. Nature of Light; wave theory, interference, polarization, significance. CHAPTER XVII INVISIBLE RADIATIONS (1) ELECTRIC WAVES AND RADIO-ACTIVITY