CARBOHYDRATES 29 



This irregular arrangement is intended to indicate that there 

 is actually a variation in the arrangement of these groups 

 around the carbon atoms in space. This arrangement is a 

 determining factor in the property of these substances known 

 as optical activity. Optical activity is the property possessed 

 by many compounds of rotating the plane of polarized light. 



If a ray of white light is passed through a crystal of Iceland 

 spar it is split into two slightly diverging rays, but this is not 

 the only change which is produced in the ray. Light is due to 

 vibration of the particles of the ether, the vibration being at 

 right angles to the direction of the ray, and in all possible 

 planes passing through the path of the ray as an axis. We 

 can picture this perhaps by imagining the cross-section of a 

 ray of light to resemble the cross-section of an orange, only with 

 many more planes of vibration. After passing through the 

 crystal of iceland spar, the light is so altered that in each of 

 the two emerging rays vibration is taking place in only one di- 

 rection. To light of this character is given the name plane 

 polarized light. It has been found that optically active sub- 

 stances have the property, if such a ray of light is passed 

 through their solutions, of rotating the plane in which the 

 ether particles are vibrating, some substances rotating the plane 

 of vibration to the right, others to the left. Not only do these 

 compounds possess this property, but a given substance, under 

 similar conditions of observation (concentration, temperature, 

 etc.) always rotates the plane of polarized light through the 

 same angle. This uniformity of behavior on the part of each 

 optically active substance gives us a most useful means of de- 

 tecting the presence of the compound in question. 



In order to have a uniform standard for comparing observa- 

 tions of optical activity it is necessary to adopt some system 

 for reporting such data. A value has been selected and given 

 the name "Specific Rotation," which is the rotation produced 

 by 1 gram of substance dissolved in 1 cubic centimeter of sol- 

 vent and the rotation observed in a tube 1 decimeter in length. 

 This value is designated by the symbol [ oc ] . 



