APPENDIX 



193 



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. 



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. 



(a)D = specific rotation for light with wave-length corresponding to 

 the D line (sodium flame). 



Then (0)0 = ± — - 

 wl 



In this formula + indicates that the substance is dextrorotatory, — that it 



is laevorotatory. 



If, on the other hand, (a)D ia known, and we wish to find the value of 



IV, then 



a 



w = 



(a)D X I' 



THE SPECTEO-POLABIMETEE 



This instrimient is one in which a spectroscope and polarising apparatus 

 are combined for the purpose of determining the concentration of substances 

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



Fig. 77. — Spectro-polarimeter of v. FleischL 



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

 difficulty arises of forming a judgment as to the identitv of two coloured 

 surfaces, as in SoleU's saccharimeter, or of two shades of the same colour, as 

 in Laurent's instrument. The light enters at the right-hand end of the 



o 



