CONTRIBUTIONS TO SCIENCE. 485 



or " Wheel of Life." In the ordinary instrument, on looking 

 through the slits in the revolving cylinder the figures are 

 seen moving on the opposite side of the cylinder. Maxwell 

 inserted concave lenses in place of the slits, the lenses being 

 of such focal length that the virtual image of the object at 

 the opposite extremity of the diameter of the cylinder was 

 formed on the axis of the cylinder, and consequently appeared 

 stationary as the cylinder revolved. 1 



In ordinary light the vibrations take place in all direc- 

 tions at right angles to that in which the light is being 

 transmitted. A beam of ordinary light is therefore sym- 

 metrical on all sides. But it is possible in various ways to 

 confine all the vibrations to one plane. Thus, if a ray be 

 reflected from the surface of polished glass at a particular 

 angle, all the vibrations take place in one plane, and it is 

 generally believed that this plane is parallel to the surface 

 of the glass, and therefore perpendicular to the plane of 

 incidence. Since all the vibrations are now taking place in 

 one plane, the beam of light has acquired, as it were, sides 

 or poles, and is said to be plane polarised. There are other 

 contrivances by which the movements may be made all to 

 take place in regular succession in a circle about the direc- 

 tion of the ray as axis, and the light is then said to be 

 circularly polarised. This may be effected by passing a 

 beam of plane polarised light through a sheet of mica (or 

 some other crystals) of a particular thickness. 



The velocity with which waves are transmitted through 

 a substance depends not only on the density but on the 

 elasticity of the substance in the direction in which the 

 vibrations take place. Now, many crystals have different 

 elasticities in different directions, and a similar condition 

 may be induced by mechanical means in other bodies. In 

 such substances the velocity of light depends on the direc- 

 tion in which the vibrations take place ; and, generally, if 

 a beam of ordinary light fall upon such a crystal it will be 

 separated into two, one of which will consist of vibrations in 



1 See Part I. p. 37. 



