122 
In this table the concentration is expressed in grammes of C,H,Br, 
present in 100 gr. of solution. The values given under “corr.” are 
the means, diminished by the correction for tbe shifting of the 
central band; it occurs here every time after + 150 scalar divisions. 
Under “calculated” are given the values satisfying the interpola- 
tion formula: 
p = 0.0009772 n — 0.0000000523 n’, 
which has been calculated from the observations according to the 
method of least squares. In this p represents the concentration 
(expressed in percentages), m the corrected number of scalar divisions. 
Hence it appears that the reproducibility of the measurements is 
+ 1 scalar division, corresponding to 0.0009 per cent. 
SUMMARY. 
The causes of the difficulties met with when it is tried to use 
the Zeiss waterinterferometer for the analysis of solutions in organic 
liquids, were discussed and the precautions were mentioned 
required to carry up the aecuracy of the measurements to the same 
order as can be reached with aqueous solutions. 
Utrecht, June 1921 van ’T Horr- Laboratory. 
