68 BACTERIOLOGICAL AND ENZYME CHEMISTEY 



consists in a wave motion of the luminiferous ether; the 

 intensity of light depends on the ampUtude of the vibra- 

 tions, the colour of the light depends on the wave length, 

 which is generally referred to as X.. 



Now in an ordinary ray of light the waves are conceived 

 of as following each other in very rapid succession, with 

 constantly varying planes of vibration ; thus, e.g., if we 

 imagine a wave motion (Fig. 11) vibrating in one instant of 

 time parallel to AB, the following wave may vibrate along 

 A'B' and the next along A"B" and so 

 „ on. Such a ray of light, therefore, 

 has no two-sidedness, that is, the 

 plane of vibration of its waves can- 

 not be determined ; on the other 

 hand, a ray of Ught aU of whose waves 

 pass through AB would be referable 

 definitely to this plane, and such a 

 ray is said to be polarised. 



The unassisted eye is unable to 

 distinguish between polarised light 

 and ordinary light ; it is conceivable 

 that if we could construct a barred 

 screen of sufficient fineness to pre- 

 vent the passage of all waves except 

 b' ^ those undulating in a plane parallel 



to the bars, we should know that the 

 light passing through the screen was 

 polarised in that plane. Now the structure of certain 

 crystals is such that they act somewhat in the manner of 

 such a screen, and compel the waves of light passing through 

 them to vibrate in defined planes. Such a crystal is tour- 

 maline ; if two pieces of tourmaline cut parallel to the long 

 axis of the crystal are placed at right angles one to the 

 other, opacity results. 



For an explanation of this property it wHl be necessary 



b" 



B 



Fig. U. 



