10 



LAURENT'S POLARIMETRE. 



[BOOK i. 



screw G is then turned till the equality is restored and the reading 

 of the circle at once gives the rotation due to the substance, right- 

 or left-handed, according as the vernier is to the right or left of the 

 zero on the scale. The following is an example of the determination 

 of the rotatory power of a solution of sodium glycocholate in alcohol. 

 The solution in a tube 2 decimetres long gave a rotation of 

 + 1 40' or r-b'66. On evaporation, 10 c.c. of the solution gave 0'322 

 grm. of dry residue, or 1 c.c. contained '0322 grm. of the salt. Now 

 the specific rotation a D being denned as that due to a column of 

 liquid 1 decimetre long and containing 1 grm. of salt per 1 c.c., we have 



+ 1'666 = * x 2 x -0322 



or 



Theory of Laurent's Polarimetre 1 . The light from the sodium flame A 

 (Fig. 4) is deprived of all traces of blue or violet rays by the potassium 

 bichromate solution in the cell B. It then passes to the doubly refracting 

 prism P, whence half of it emerges polarized in one plane, the other half, 

 polarized in a perpendicular plane, being refracted away from the axis and 

 stopped by a diaphragm. 



Fig. 4. DIAGRAM OF LAURENT'S POLARIMETRE. 



At D is a diaphragm of which one half is covered by a plate of quartz 

 cut with the axis in the surface and parallel to the edge 2 . To understand 

 the effect of this crystal let Fig. 5(1) represent the diaphragm, the shaded 

 part being the quartz plate. Let OB be the direction of vibration of 

 the light after polarization by the prism. This will still continue to be the 

 direction of vibration of the light which goes through the right half of the 

 diaphragm, but a ray vibrating parallel to OB will on entering the quartz 

 on the left be resolved into two rays, one vibrating parallel to the axis 

 OA, which we represent by Oy, the other perpendicular to the axis, which 

 we represent by Ox. These two rays will travel at different rates through 



1 For this account of the theory of Laurent's Polarimetre, I am indebted to my 

 fiicnd Mr J. H. Poynting, M.A., Fellow of Trinity College, Cambridge. 



2 When cut in this manner quartz has no rotatory power but behaves just as any 

 other uniaxal crystal. 



