THE GROWTH OF PHYSICAL IDEAS, TOr 



double refraction of Iceland spar and had found tFiat both 

 rays are polarized, the planes of polarization being at right 

 angles to each other. Fresnel's theory explained both inter- 

 ference and polarization and gave the mathematical relations 

 for all these phenomena. Fresnel thought of the vibrations 

 of light as vibrations of the ether, which now assumed con- 

 tradictory qualities because the great velocity of light made 

 necessary the idea that the ether was a solid of enormous 

 rigidity, while at the same tiine it imposed no resistance to 

 the passage of matter such as the planets. 



Much of the theoretical work of the nineteenth century 

 was concerned with the discussion of the properties of the 

 ether and its relation to matter, but the gi^eatest advance in 

 the whole theory of radiation came with the suggestion in 

 1864 by J. Clark Maxwell that light w^as an electromagnetic 

 phenomenon. Maxwell investigated mathematically the 

 propagation of electric and magnetic forces in space and 

 found the velocity of propagation to be identical with the 

 known velocity of light and the calculated properties— those- 

 actually exhibited by light. He showed that in electromag-- 

 netic waves the electric and magnetic vibrations occur at right, 

 angles to each other and to the direction of the ray, ^vhich is,, 

 of course, normal to the waves of light, and that electromag-. 

 netic waves would be capable of being polarized and w;ould 

 show the phenomena of refraction, reflection, and interfer- 

 ence. Thus he considered light an electromagnetic phenome- 

 non corresponding to a restricted range of wave lengths; and 

 he concluded that longer waves might exist which were far 

 too long to be seen by the eye but could conceivably be 

 detected by other means. 



This theory was confirmed experimentally by Heinrich 

 Hertz in 1887, and the electric waves discovered by him are 

 those now used in radio communication. The w^hole range 

 of electromagnetic radiation between the radio waves, many 

 meters long, and the waves of light has been generated and 

 observed. Moreover, the discovery of waves shorter than 

 those of visible light, known as ultraviolet waves , was fok 



