THE SIZE OF ATOMS. 201 



very small ranges 1 parallel to one line. If you 

 place your eye in the plane through the length 

 of the tube and perpendicular to that line, you 

 will see light from all parts of the tube, and 

 this light which you see will consist of vibrations 



1 In the following question of the recent Smith's Prize Exam- 

 ination at Cambridge (paper of Tuesday, Jan. 30, 1883), the 

 dynamics of the subject, and particularly the motion of the ether 

 produced by keeping a single spherule embedded in it vibrating 

 to and fro in a straight line, are illustrated in parts (a) and (d) : 



' ' 8. (a) From the known phenomenon that the light of a 

 cloudless blue sky, viewed in any direction perpendicular to the 

 sun's direction, is almost wholly polarised in the plane through the 

 sun, assuming that this light is due to particles of matter of diameters 

 small in comparison with the wave-length of light, prove that the 

 direction of the vibrations of plane polarised light is perpendicular 

 to the plane of polarisation. 



"(b) Show that the equations of motion of a homogeneous 

 isotropic elastic solid of unit density, are 



(Pa d5 





#r = ( k 



where k denotes the modulus of resistance to compression ; n the 

 rigidity-modulus ; a, ft, 7, the components of displacement at (x, y, 



z, /); and 



da d$ dy 



