100 THE PATH OF SCIENCE 



They involve a periodic disturbance moving along the rays 

 from wave surface to wave surface. Two rays or two waves 

 coming from the same point source can be united in a point 

 in such a way that the maximum of one wave ^vill coincide 

 with the minimum of the other and so neutralize it. Thus 

 two waves of light can, under certain circuinstances, produce 

 darkness. This idea was given definite form in 1801 by 

 Thomas Young. Before this, the corpuscular theory of light 

 had been generally accepted for almost a century, largely be- 

 cause it had been sponsored by Isaac Newton. Young 

 founded his views on the nature of light on the following 

 hypotheses: 



A luminous body produces ^vaves in a medium, the ether, 

 w^hich pervades the entire universe. Different colors of light 

 owe their differences to the frequency of their vibrations, 

 which produce different sensations in the retina. The pro- 

 duction of darkness by the mutual action of two waves of 

 light Young described as interference; and he was able to 

 measure and explain by the wave theory both the diffraction 

 fringes discovered by Grimaldi and the colors of thin films 

 discovered by Newton. Young measured the length of the 

 waves of light, finding that the limit of the spectrum in the 

 red corresponded to waves about 0.0007 millimeter long, 

 while the violet rays at the other end of the spectrum had a 

 length of 0.0004 millimeter. 



Young's theory was improved by Augustin Fresnel, who 

 considered that the waves moving along the rays ^vere trans- 

 verse waves, vibrations in the plane perpendicular to the 

 path of the light. This made it possible to explain not only 

 diffraction and interference but also the phenomenon of 

 polarization, which had been discovered in 1809 by E. L. 

 Malus, a French physicist, who had observed that light re- 

 flected by a mirror at an incidence angle of about 57° is 

 totally polarized, that is, it has vibrations only in the direc- 

 tion normal to the ray in the plane of reflection. A second 

 reflection from a plane at right angles to the first will extin- 

 guish the light. Malus had then directed his attention to the 



