322 PROGRESS IN "PHYSICS. 



that light was a vibratory disturhanoe transmitted by means of waves 

 necessitated the assumption of the existence of such a medium through- 

 out all space in which light traveled. What is known as the ethereal 

 medium, at first a purely imaginary substance, but whose real exist- 

 ence is practically established, satisfies this demand, and the hypothesis 

 that light is transmitted by waves in such a medium, originating in a 

 vi))ratory disturbance at the source, has been of inestimable valu(^ to 

 physical science. 



The work of Thomas Young was done in the very first years of the 

 nineteenth century. He was for two years professor of natural phi- 

 losophy' in the Royal Institution just founded by Count Rumford, and 

 was the first to fill that chair. In 1801, in a paper presented to the 

 Royal Societ}', he argued in favor of the undulatory theor}^, showing 

 how the interference of waves would explain the color of thin plates. 

 His papers were not for several years received favorably, and they 

 were severely criticised by Lord Bi-ougham. Augustus Fresnel fol- 

 lowed Young, ])ut quite independently, about ten years later, and b}' 

 him the undulatory theory received elaborate experimental and math- 

 ematical treatment. 



In the meantime another Frenchman had made a capital discovery 

 in optics, which seemed at first to be quite incompatible with the wave 

 theory. This was the discovery of what is known as polarization of 

 light, by Malus, a French engineer, who hit upon it while investigating 

 double refraction of crystals, for a study of which the French Institute 

 had ofi'ered a prize in 1808. Malus found that when light fell upon a 

 surface of glass at a certain angle a portion of the reflected light 

 appeared to have acquired entireh^ new properties in regard to further 

 reflection, and the same was true of that part of the beam which was 

 transmitted through the glass. The light thus aflfected was incapable 

 of further reflection under certain conditions and as the beam seemed 

 to behave diflerently according to how if was presented to the reflect- 

 ing surface the term polarization was applied to the phenomenon. It 

 was found that the two rays into which a single beam of light was split 

 by a doubly refracting crystal (a phenomenon which had long been 

 known) were affected in this way, and that light was polarized by 

 refraction as well as by reflection. Malus was a believer in the cor- 

 puscular theory of light; but it was shortly proved, first by Thomas 

 Young, that the phenomenon of polarization was not onl}- not opposed 

 to the wave theory, but that that tht^ory furnished a rational explana- 

 tion of it. This explanation, in brief, assumes that ordinary light is 

 a wave produced by a vibratory motion confined to no particular plane, 

 the direction of vibration being at right angles to the direction of the 

 wave and in any or in rapid succession in all azimuths. When light 

 is polarized, the vibratory motion in the ether is restricted to one par- 

 ticular form, a line if plane polarized, a circle or an ellipse if circu- 



