APPENDIX 



229 



illuminated : in any other position, or if rotation has been produced by the 

 solution when the nicols have been set at zero, the two halves appear un- 

 equally illuminated. This is corrected by means of a rotation of the analyser 

 that can be measured in degrees by a scale attached to it. 



The specific rotatory power of any substance is the amount of rotation in 

 degrees of a circle of the plane of polarised light produced by 1 gramme of 

 the substance dissolved in 1 c.c. of liquid examined in a column 1 deci- 

 metre long. 



If a = rotation observed. 



w - weight in grammes of the substance per cubic centimetre. 

 I = length of tube in decimetres. 



[o] D = specific rotation for light with wave-length corresponding to 

 the D line (sodium flame). 



Then 



wl 



In this formula + indicates that the substance is dextro-rotatory, that it 

 is Isevo-rotatory. 



If, on the other hand, [] is known, and we wish to find the value of w, 

 then 



w 



THE SPECTRO-POLARIMETER 



This instrument is one in which a spectroscope and polarising apparatus 

 are combined for the purpose of determining the concentration of substances 



FIG. 77. Spectro-polarinieter of v. Fleischl. 



which rotate the plane of polarised light, It was invented by E. v. Fleischl 

 for the estimation of sugar in diabetic urine. Its chief advantage is that no 

 difficulty arises in forming a judgment as to the identity of two coloured 



