ON THE POLARIZATION OF LIGHT. 245 



the object as indicated by the various colours occasioned by 

 the different densities of its parts. A list of those objects that 

 win exhibit the most beautiful series of colours will be given 

 in another part of the work. 



It may be here stated, that the film of selenite employed to 

 give coloiu: to objects, should be mounted between two glasses 

 for protection ; it may be even immersed in Canada balsam, in 

 the same manner as an ordinary object. Some persons employ 

 a large film mounted in this way between plates of glass, with 

 a raised edge to act as a stage for supporting the object on, it 

 is termed the " selenite stage." 



Cause of the Colours of Polarized Light. — In fig. 153 it was 

 shown, that when a beam of light reflected from a plate of 

 glass, at the angle of 56" 45', was received by another plate of 

 glass at the same angle, it would be found (if this second plate 

 were capable of being revolved) that in two positions in one 

 revolution the light would be entirely stopped, whilst in two 

 others it would be whoUy reflected; in the microscope the 

 same effect can be shown by two prisms, one either above the 

 eye-piece or between it and the object-glass, and the other 

 between the object-glass and the mirror. It has been stated 

 before, that if a thin plate of a doubly refracting crystal were 

 placed between the prisms (when in the situation that the 

 transmitted ray of polarized light from the lowest prism is 

 stopped by the upper prism), it would not only cause the light 

 to pass through the first prism, but, according to the thickness 

 of the plate of the crystal, so should we have colours comple- 

 mentary of each other, or which, together, would make white 

 light. From a very early period, philosophers have been 

 employed in the investigation of the nature of light, and two 

 principal theories have been advanced, one by Newton, who 

 maintained that light is material, and is emitted by all self- 

 luminous bodies in minute particles ; the other by Huyghens, 

 who supposed that an elastic medium, called ether, fills aU 

 space, and occupies the intervals between the particles of aU 

 substances, and that luminous bodies excite vibrations in this 

 ether, which spread by waves, in the same manner as those 

 formed in water when a foreign substance, such as a stone, is 

 dropped into it. This latter theory is the one now generally 



