PHYSICS OF THE NINETEENTH CENTURY. LIGHT. 463 



on which the hypothesis is justified ; but one or two familiar illustra- 

 tions may serve both to fix the conception and to lessen its difficulty. 

 A rope fixed at one end and held in the hand at the other, may by a 

 jerk of the hand be thrown into a series of waves, which are the result 

 of lateral vibrations of its parts, and these waves travel along the rope. 

 Another illustration of lateral disturbance lineally transmitted is some- 

 times shown at lectures. A series of magnetic needles are arranged 

 in the magnetic meridian, and when the pole of a permanent magnet 

 of the same name as the outer pole of the last needle is brought near 

 to the latter, the repulsion causes the needle to turn aside, and this 

 movement is followed by the rest of the needles one after the other, 

 on account of their mutual action. The action is progressive by pass- 

 ing along the line, and the movements of each needle are lateral only. 



The undulatory hypothesis explains the phenomena of interference, 

 refraction, etc., equally well whether the vibrations are conceived as 

 taking place in the direction of the ray or transversely. Polarization 

 is satisfactorily accounted for only on the latter hypothesis, but a cer- 

 tain relation between interference and polarization was discovered by 

 Fresnel and Arago, and admitting undulations at all, it would appear 

 impossible to explain the fact we here allude to otherwise than by the 

 conception of transverse vibrations. 



Fresnel applied his great mathematical power to the development 

 of Young's theory of interferences, and to the more complete in- 

 vestigation of the phenomena of diffraction. "When light enters a dark 

 room through a very small aperture, the shadows of bodies in the 

 room so lighted, instead of being bounded with a sharp edge, are sur- 

 rounded by variously coloured bands. Also if the opaque body is 

 narrow enough and its shadow is received at some distance, the interior 

 of the shadow, where complete darkness would be expected, may be 

 observed divided into equal spaces by alternate dark and bright bands, 

 which last are, like those that surround the shadow, coloured. Fresnel 

 fixed in his -window-shutter a piece of copper foil, in which a very small 

 hole was made, and upon this hole the rays of the sun were concen- 

 trated by a large lens placed outside of the window. Finding this 

 arrangement inconvenient on account of the sun's motion, he replaced 

 the lens by a mirror which reflected the solar rays in the required direc- 

 tion, and instead of the very small hole in the copper foil, a lens of a 

 short focal length, half an inch or less, was placed in an opening in the 

 shutter. In the focus of this lens the rays were concentrated in a 

 point which became a source of light equivalent to a very small aper- 

 ture. In his first trials Fresnel, not having at hand a glass lens of 

 sufficiently short focus, improvised one by placing a drop of honey 

 in a round hole in a thin plate of copper. A piece of iron wire illu- 

 minated by the light which passed through the honey-drop gave well- 

 defined fringes in its shadow, permitting excellent observations and 

 measurements to be made. At first Fresnel studied the fringes by 



