26 



PRACTICAL PHYSIOLOGY. 



[Til. 



If a = the observed rotation ; 



^? = the weight in grams of the active substance contained in i cc. of 



liquid ; 



Z=the length of the tube in decimetres ; 

 () D = the specific rotation for light corresponding to the light of a 



sodium flame ; 

 then 



The sign + or - indicates that the substance is dextro- or laevo-rotatory. 

 Various instruments are employed. Use 



Laurent's Polarimeter. This instrument is a so-called "half-shadow 

 polarimeter," and must be used in a dark room (fig. 12). 



M 



FlG 12. Laurent's Half-Shadow Polarimeter. 



18. Determination of the Specific Rotatory Power of Dextrose. 



(n.} Fill one of the decimetre tubes with distilled water, taking care that 



