66 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



The Theory of the Polarimeter 



Light is one of the primary forms of energy which reaches 

 this earth from the sun. It is well known that the atmosphere 

 of air surrounding the earth becomes more and more attenuated 

 as the distance from the earth increases, and, in fact, does not 

 extend even in a rarefied form to a greater distance than 

 approximately 200 miles from the surface of the earth ; some 

 medium other than the atmosphere must, therefore, be con- 

 ceived of as a means of transmitting light and other forms of 

 energy from the sun to the earth. This medium, which is 

 thought of as filling all space, has been termed the luminiferous 

 ether, for the reason just mentioned. Light is conceived of 

 physically as a wave motion set up in this all-pervading ether. 

 The essential features of wave motion can be readily studied 

 by carefully observing the ripples formed when a pebble is 

 thrown into a still pool of water ; it will be seen that the 

 water does not move as a whole from the point where the 

 pebble plunged, but that a series of up and down motions 

 takes place in successive portions of water. This can be 

 easily verified by throwing in a few light match stalks, which 

 will be seen merely to move up and down and not to approach 

 the edge to any appreciable extent. The same essential 

 feature of wave motion is clearly seen when the wind blows 

 over a field of wheat ; obviously, here individual ears of wheat 

 cannot move beyond certain limits, and there is only a to 

 and fro, or up and down movement. The regular movement 

 within certain limits, such as the water particles or the ears of 

 wheat exhibit under the above circumstances, is known as a 

 vibration ; the extent of the displacement of any given vibrat- 

 ing particle from its position of rest is known as the amplitude 

 of the vibration, and a motion such as we have been con- 

 sidering is known generally as wave motion. 



A wave length is measured from crest to crest or from 

 hollow to hollow of the wave. If it were possible to set two 



