Nipher — Physics During the Last Century. 115 



tablished a period for sun-spot frequency. At once Sabine 

 in England, Gautier in France, and Wolf in Switzerland, 

 pointed out, independently of each other, the coincidence of 

 sun-spot maxima and those of magnetic oscillation. On Aug. 

 3, 1872, Young observed at Sherman in the Kocky Mountains, 

 three immense solar disturbances at intervals of about an 

 hour, and the magnetic needle at that station was deflected 

 entirely off the scale. The coincidence of the two phenomena 

 could not be established because the time of the magnetic dis- 

 turbance was not noted. But the Greenwich and Stonyhurst 

 photographic records showed that the magnetic disturbances 

 in England were felt at the same times that Young saw the 

 luminous outbursts, where hundreds of dark lines in the spec- 

 trum were suddenly reversed for a few minutes at a time. 



In the meantime Maxwell had been putting the ideas of 

 Faraday into mathematical language. In his great treatise 

 which appeared in 1873, he developed the idea that light was 

 an electromagnetic induction, differing from that which an 

 alternator may produce only in wave frequency, or wave 

 length This view of the subject gave a complete explana- 

 tion of the experimental results of Fresnel, according to 

 which the so-called vibrations of light were in a plane at right 

 angles to the direction of propagation of the ray. It also 

 linked with the discovery of Oersted in 1820, that a magnetic 

 line of force of a linear current is a circle, having some point 

 on the current as a center. According to Maxwell, the elec- 

 trical and magnetic lines of force, which are thus shown to 

 be at right-angles to each other, are components of luminous 

 vibration. Notwithstanding the long perspective of prior 

 evidence tending to corroborate this view, Maxwell's ideas 

 did not at first receive very general assent. But the electro- 

 magnetic theory of light has been steadily reinforced by every 

 subsequent development. We can now see that the induced 

 discharges which occur here and there on conductors remote 

 from a great electrical discharge, are the spray over the 

 sunken rocks, or the splashing surf along the shores of an 

 ethereal ocean. It was Hertz who in 1888 first produced 

 and studied by electrical means these ether waves which 

 serve as the messengers in the wireless telegraphing of to- 



