162 



Biological Chemistry. 



was optically inactive. This acid was known as " racemic 

 acid." The first great classical research of Pasteur deals 

 with the relationship between racemic and tartaric acids 

 (1848-54), but before considering these results it will 

 be necessary to diverge for a time to discuss the meaning 

 of the expression " optical activity," to which repeated 

 reference has already been made. 



Optical Activity. If a ray of light is allowed to fall 

 on to a rhomb of Iceland spar (a certain crystalline form 

 of calcium carbonate) part will pass straight through, 

 whereas another part will be refracted and travel through 

 another path. A double image will therefore be seen of 

 any object viewed through a crystal of the spar. This 

 phenomenon is known as double refraction, and is due to 

 the anisotropic properties of the crystal that is, the 

 physical properties of the crystal are different in different 

 directions. Of the two rays of light which pass through 

 the spar, one obeys the ordinary laws of refraction, and is 



known as the " ordinary ray " ; 

 whereas the other does not, and 

 is called the " extraordinary ray." 

 By means of a device known 

 as the Nicol prism, it is possible 

 so to arrange the light incidence 

 * that only the extraordinary ray 

 passes through. The Nicol prism 

 is constructed in the following 

 manner: A rhomb of Iceland 

 spar is cut in two by a plane 

 AC perpendicular to the prin- 

 cipal plane for the face AB 

 (see Fig. 18). The two cut 

 surfaces are then polished and cemented together by 

 Canada balsam. The refractive index of Canada balsam 



Fig. 18. 



