OPTICS. 603 



of color in a pure spectrum ; but this I believe to be a subjective phe- 

 nomenon depending upon contrast." 



CHAPTER XIII. 



UNDULATOET THEORY. 



Direction of the Transverse Vibrations in Polarization. 



IN the conclusion of Chapter xiii. I have stated that there is a point 

 in the undulatory theoiy which was regarded as left undecided by 

 Young and Fresnel, and on which the two different opinions have been 

 maintained by different mathematicians; namely, whether the vibra- 

 tions of polarized light are perpendicular to the plane of polarization 

 or in that plane. Professor Stokes of Cambridge has attempted to 

 solve this question in a manner which is, theoretically, exceedingly 

 ingenious, though it is difficult to make the requisite experiments in a 

 decisive manner. The method may be briefly described. 



If polarized light be diffracted (see Chap. xi. sect. 2), each ray will be 

 bent from its position, but will still be polarized. The original ray and 

 the diffracted ray, thus forming a broken line, may be supposed to be con- 

 nected at the angle by a universal joint (called a Hoove's Joint], such 

 that when the original ray turns about its axis, the diffracted ray also 

 turns about its axis ; as in the case of the long handle of a telescope 

 and the screw which is turned by it. Now if the motion of the origi- 

 nal ray round its axis be uniform, the motion of the diffracted ray 

 round its axis is not uniform : and hence if, in a series of cases, the 

 planes of polarization of the original ray differ by equal angles, in the 

 diffracted ray the planes of polarization will differ by unequal angles. 

 Then if vibrations be perpendicular to the plane of polarization, the 

 planes of polarization in the diffracted rays will be crowded togethei 

 in the neighborhood of the plane in which the diffraction takes place, 

 and will be more rarely distributed in the neighborhood of the plane 

 perpendicular to this, in which is the diffracting thread or groove. 



On making the experiment, Prof. Stokes conceived that he found, 

 in his experiments, such a crowding of the planes of diffracted polari- 

 zation towards the plane of diffraction ; and thus he held that the 



